Functional Nanostructured Materials: Synthetic

Aspects & Properties Evaluation

Kumulative Dissertation

zur

Elangung des Doktorgrades

der Naturwissenschaften

(Dr. rer. nat.)

dem

Fachbereich Chemie der Philipps-Universität Marburg

Vorgelegt von

M. Sc. Chem. Fei Chen

aus

Jiangxi / China

Marburg an der Lahn 2011

Vom Fachbereich Chemie der Philipps-Universität Marburg am

___________ als Dissertation angenommen.

Erstgutachterin: Prof. Dr. Seema Agarwal

Zweitgutachter: Prof. Dr. Andreas Greiner

Tag der mündlichen Prüfung: ______________

Hochschulkennziffer: 1180

"To study without thinking is useless, to think without studying is idle."

-- Confucius

I  

Table of Contents

List of symbols and abbreviations .................................................. 1 

1. Introduction ................................................................................. 5 

1.1  Motivation .............................................................................................................. 5 

1.2  Theoretical background and prior research ............................................................ 5 

1.2.1  Smart nano-objects by self-assembly of block copolymers in solution ...... 5 

1.2.2  Functional nanoparticles by heterophase polymerization method .............. 8 

1.2.3  Composite nanomaterials: synthesis by electrospinning and their applications ................................................................................................................ 9 

1.3 Aim and concept of this work ................................................................................... 20 

2. Summary in German (Zusammenfassung) ............................. 22 

3. Summary .................................................................................... 25 

4. Cumulative part of dissertation ............................................... 27 

4.1 Stimuli-Responsive Elastic Polyurethane-Based Superabsorber Nanomat Composites 27 

4.1.1 Summary and discussion ................................................................................ 27 

4.1.2 Declaration of my contribution ....................................................................... 31 

4.2 Multifunctional Polyurethane Aqueous Dispersions showing Thermo Responsivity with UCST and Antibacterial Properties ......................................................................... 32 

4.2.1 Summary and discussion ................................................................................ 32 

4.2.2 Declaration of my contribution ....................................................................... 38 

4.3 A Fast Degrading Odd-Odd Aliphatic Polyester-5,7 made by Condensation Polymerization for Biomedical Applications .................................................................. 39 

II  

4.3.1 Summary and discussion ................................................................................ 39 

4.3.2 Declaration of my contribution ....................................................................... 44 

4.4 Nanofibers by Green Electrospinning of Aqueous Suspensions of Biodegradable BlockCopolyesters for Applications in Medicine, Pharmacy and Agriculture ............... 45 

4.4.1 Summary and discussion ................................................................................ 45 

4.4.2 Declaration of my contribution ....................................................................... 49 

4.5 Low Dielectric Constant Polyimide Nanomats by Electrospinning .......................... 50 

4.5.1 Summary and discussion ................................................................................ 50 

4.5.2 Declaration of my contribution ....................................................................... 55 

5. Outlook ....................................................................................... 56 

6. Acknowledgements .................................................................... 58 

7. Literature ................................................................................... 61 

8. Appendix .................................................................................... 67 

8.1 Publication “Stimuli-Responsive Elastic Polyurethane-Based Superabsorber Nanomat Composites” ..................................................................................................... 68 

8.2 Manuscript “Multifunctional Polyurethane Aqueous Dispersions showing Thermo Responsivity with UCST and Antibacterial Properties” ................................................. 76 

8.3 Publication “A Fast Degrading Odd-Odd Aliphatic Polyester-5,7 made by Condensation Polymerization for Biomedical Applications”........................................ 101 

8.4 Publication “Nanofibers by Green Electrospinning of Aqueous Suspensions of Biodegradable BlockCopolyesters for Application in Medicine, Pharmacy and Agriculture” ................................................................................................................... 121 

8.5 Publication “Low dielectric constant polyimide nanomats by electrospinning” ..... 129 

1  

List of symbols and abbreviations AA Acrylic acid

AC 4,4'-Di(methacryloylamino) azobenzene

AM Acrylamide

BPDA 3,3′,4,4′-biphenyltetracarboxylic dianhydride

BTDA Benzophenone-3,3′,4,4′-tetracarboxylic dianhydride

CMC Critical micelle concentration

DABCO 1,4-Diazabicyclo[2.2.2]octane

DCM Dichloromethane

d6-DMSO Deuterated dimethylsulfoxide

DEMA Diethanol-N-methylamine

DLS Dynamic light scattering

DMF Dimethylformamide

DMTA Dynamic mechanical thermal analysis

DSC Differential scanning calorimetry

6FDA 4,4′-(Hexafluoroisopropylidene)diphthalic

anhydride

FTIR Fourier transform infrared spectroscopy

GPC Gel permeation chromatography

HMBC Heteronuclear multiple bond correlation

2  

HMQC Heteronuclear multiple quantum correlation

MBC Minimum bactericidal concentration

mg Milligram

MIC Minimum inhibition concentration

ml Milliliter

Mn Number average molecular weight

Mw Weight average molecular weight

MWDs Molecular weight distributions

nm Nanometer

NRs Nanorods

PA6 Polyamide 6

PAA Poly(amic acid)

PDI Polydispersity

PBS Phosphate buffered saline

PCL Poly(ε-caprolactone)

PEG Polyethylene glycol

PEO Polyethylene oxide

PHA-b-PEO Poly(hexamethylene adipate)-Polyethylene

oxide block copolymers

PI Polyimide

PMDA Benzene-1,2,4,5-tetracarboxylic dianhydride

3  

Polyester-5, 7 Poly(pentylene heptanoate)

PPA Polyphosphoric acid

P2VP Poly(2-vinylpyridine)

PSAPs Photochromic Superabsorbent Polymers

QD Quantum dots

SAP Superabsorbent polymers

SEM Scanning Electron Microscopy

SILAR Successive ionic layer adsorption and reaction

Span 60 Sorbitan monostearate

T Temperature (°C)

TDI 2,4-toluenediisocyanate

TE Tissue engineering

TEM Transmission electron microscopy

Tg Glass transition temperature

TGA Thermal gravimetric analysis

THF Tetrahydrofuran

THz-TDS Terahertz time-domain spectroscopy

Ti(OBu)4 Titanium(IV) butoxide

Tm Melting temperature

TMS Tetramethylsilane

4  

TPU Thermoplastic polyurethane

UCST Upper critical solution temperature

μ Micrometer

λ Wavelength

UV/Vis Ultraviolet-visible spectroscopy

WAXD Wide angle X-ray diffraction

∆Hm Heat of fusion

∆Hc Heat of crystallization

ΔHm° Heat of fusion with 100% crystallinity

   

5  

1. Introduction 1.1 Motivation

Functional nanomaterials with size less than 1μm have received extensive scientific as

well as technological attention due to their potential applications in microelectronics,

biomedical, and optical materials. This is because new and unexpected properties are

less likely to develop with micron-scale bulk materials. There are many methods to

prepare functional nanomaterials including phase separation, block copolymer

self-assembly, electrospinning and heterophase polymerization etc. Divided by

morphology and shape, functional nanomaterials comprise nanofibers, nanoparticles,

naorods, nanowires etc. Among these nanomaterials, nanofibers and nanoparticles

have received increasing attention due to their potential applications in biomedical

and microelectronics fields.

Electrospinning is the state-of-the-art method for the preparation and production of

continuous nanofiber nonwovens, which has been highlighted in recent years. Due to

the small size, high surface to volume ratio and other unique properties, a wealth of

chemistry as well as new methods have been applied to modify the electrospun

nanofibers to impart new functionalities. On the other hand, heterophase

polymerization provides chemists a direct way to form functional nanomaterials

(mainly nanoparticles), whose properties and morphologies can be controlled by

polymerization methods and conditions.

This work aims to prepare some functional nanomaterials including

stimuli-responsive nanofibers and nanoparticles, biodegradable nanofibers and low

dielectric constant nanofibers for biomedical and microelectronic applications.

1.2 Theoretical background and prior research

1.2.1 Smart nano-objects by self-assembly of block copolymers in solution

The strategies to synthesize functional nanomaterials (mainly polymer materials)

contain a broad range of methodology, such as phase separation[1,2], heterogeneous

polymerization to form nano (micro) particles[3,4], electrospinning of polymer

6  

solutions to form nanofibers[5-9] and nano-objects by self-assembly of block

copolymers in solution[10]. Among these different methods, block copolymers

occupy a huge area of research, because they offer a vast range of possibilities for

architecture, size, and chemical composition. Advances in polymer chemistry[11],

such as anionic polymerization and most recently living radical polymerization[12],

have enabled a large amount of block copolymers to be synthesized with great control

over their architecture, molecular weight, chemical composition and functionality.

Their intrinsic properties allow the combination of different polymers and therefore

the design of novel materials potentially comprising several different properties (e.g.

thermoplastic, rubber, electrical conductivity etc.).

Amphiphilic molecules in water are the most studied examples of self-assembling

molecules in selective solvents. The block copolymers undergo two basic processes in

solvent media: micellization and gelation. Micellization occurs when the block

copolymer is dissolved in a large amount of a selective solvent for one of the blocks.

Under these circumstances, the polymer chains tend to organize themselves in a

variety of structures from micelles or vesicles to cylinders. The soluble block will be

oriented towards the continuous solvent medium and become the “corona” of the

micelle formed, whereas the insoluble part will be shielded from the solvent in the

“core” of the structure (see Figure 1). In contrast to micellization, gelation occurs

from the semidilute to the high concentration regime of block copolymer solutions

and results from an arrangement of ordered micelles.

7  

Figure 1. Different geometries formed by block copolymers in selective solvent

conditions: (A) micellization at the critical micelle concentration and (B) gelation at

high concentration from diblock copolymers.

Thanks for the development of polymerization methods, a variety of monomers were

chosen to form amphiphilic block copolymer micelles, in which stimulus responsive

nano-assemblies attracted great attention these years. By design the specific

hydrophobic polymers to external stimulus such as pH[13], temperature[14], and

hydrolytic degradation[15], block copolymers micelles and vesicles, therefore find

applications for the delivery of anticancer drugs[16] and as contrast agents for

medical imaging[17] and so on. Lecommandoux and co-workers employed

stimuli-responsive blocks to encapsulate various hydrophilic and /or hydrophobic

species, such as drugs, in these vesicles and used them as efficient carriers that can

deliver their contents at the right place and moment by activation of magnetic or pH

triggers[18].

In brief, by controlling the architecture of individual molecules, amphiphilic block

copolymers can generate nanostructures either in an undiluted melt or in solution.

8  

Their synthetic nature allows the design of interfaces with different chemical

functional groups and geometrical properties which will undoubtedly find a wide

range of applications in the scientific as well as technical community. Still it has some

drawbacks, such as complexity of choosing proper solvent and determining the

critical micelle concentration (CMC) for micelle formation. Research on block

copolymer nanoparticles and nanostructures is a relatively new area and there is still a

great deal left to explore.

1.2.2 Functional nanoparticles by heterophase polymerization method

Compared with self-assembly method utilizing resultant amphiphilic block

copolymers to form nanomaterials in aqueous solution, heterophase polymerization is

a direct way to form nanoparticles or other nanostructured materials in water during

heterophase polymerization process[19]. As one of the most important techniques to

prepare polymer latices which play an essential role in our daily life such as cosmetics,

detergents, newspapers, or paints, the development of new heterophase

polymerization techniques, the adoption of new polymerization mechanism to the

particular conditions, the synthesis of block copolymers and hybrid nanoparticulate

structures were given special emphasis in recent years.

Among all the heterophase polymerization methods, emulsion polymerization is the

most relevant, but also special cases, as the particles are newly built up by nucleation

processes and monomer transport via the continuous phase. The monomer can be fed

continuously into the reactor either as neat monomer or as an emulsion. For large

scale production in industry, the monomer to water ratio is adjusted in such a way that

a solids content typically between 40- 60 % or even higher is obtained.

In the cases of suspension, miniemulsion and microemulsion polymerization, the

monomer must be only slightly water soluble as it has to form a separate phase in the

shape of spherical droplets where size is controlled by a proper choice of the

dispersing technique (stirring, ultrasonic treatment, homogenization) in combination

with the stabilizing system. The droplet size decreases in the order suspension>

9  

microsuspension> miniemulsion> microemulsion polymerization. The polymerization

recipes are designed in such a way (for instance oil soluble instead of water soluble

initiators) that the polymerization takes place mainly inside the preformed monomer

droplets. In these techniques selected and specific stabilizers have to support the

emulsification process and the stabilization of the monomer droplets. The comparison

of different heterophase polymerization processes and mechanisms were well

summarized in a previous review by Antonietti[3].

As a promising synthetic way to synthesize nanostructured functional materias, the

controlled radical polymerization (CRP) has become one of the most rapidly growing

topics in the field of polymer research[20]. Thus enabled by the adaptation of

controlled radical polymerization processes to heterophase polymerization processes,

it is straightforward to synthesize well defined polymers and also block copolymers

under aqueous heterophase conditions.[21] This is an important step towards a broad

commercial applications of controlled radical polymerization as all the benefits of

both heterophase polymerization process and CRP can be combined.

In summary, both from an application point of view as well as scientific perspective,

heterophase polymerization provides chemists and material scientists a powerful and

challenging technique to prepare nanostructured functional materials. Many novel

well-controlled morphology and chemical composition with new properties or

functions (micro) nanoparticles will be brought by such promising polymerization

method.

1.2.3 Composite nanomaterials: synthesis by electrospinning and their

applications

As discussed in previous chapters, self-assembly and heterophase polymerization

provide material chemists huge opportunities to fabricate functional nanostructures

with well defined morphology and chemical composition. One limitation for these

two methods in common is that the nanostructured materials can only be produced in

aqueous/organic phase and morphology is mainly micro (nano) particles. Some

10  

methods such as phase separation can also prepare one dimensional (1D)

nanostructured materials like nanorods, nanowires, but among these methods,

electrospinning seems to be the simplest and most versatile technique capable of

generating 1D nanostructures (mainly nanofibers, other types of 1D nanostructures,

such as nanobelts or nanorods can be prepared by special methods[22]) from a variety

of polymers and composite materials[6, 7, 23]. The simplicity to control the fiber

diameter, the high surface to volume ratio, and pore size and wide variety in usable

polymers, make electrospinning attractive for a wide range of biomedical applications

such as tissue engineering, wound dressing, drug delivery[9] and some other

application in filters[24], catalysis[25], nanofiber reinforcement[26]and so on.

1.2.3.1 Electrospinning process

The typical setup for electrospinning generally consists of a high voltage power

supply, a spinneret, and an electrically conductive collector (like a piece of aluminum

foil) (Figure 2). During electrospinning, due to the surface tension, a droplet of

polymer solution is formed at the tip of the needle, the applied voltage make the

polymer solution highly electrified and the induced charges are evenly distributed

over the surface. Once the strength of the electric field overcomes the surface tension

of the liquid, a liquid jet forms and moves towards the counter electrode[27]. During

the movement of the jet in the air, the remaining solvent evaporates and solid fibers

with certain diameters are deposited on the collector. A variety of parameters such as

polymer concentration, applied voltage, solution conductivity, solvent, temperature

and humidity etc. will influence the resultant fibers[28], there are no universal

parameters applicable to every polymer, but by adjusting some key parameters such

as polymer concentration, applied voltage and humidity, a vast range of

nanostructured materials (mainly nanofiber) can be formed with different diameters,

shape and morphology.

11  

Figure 2. Schematic setup for electrospinning.

1.2.3.2 Methodology for fabrication of composite nanomaterials

Emulsion electrospinning

As a versatile and convenient technique, conventional electrospinning has already

proved to be a promising technique to form nanofibers for future biomedical

applications, especially controlled drug delivery. By directly electrospinning of

mixtures of drugs and polymer, hydrophobic drugs could be encapsulated into

hydrophobic nanofibers (like PLLA nanofiber) but showed nearly zero-oder kinetics

of drug release[29]. On the other hand, water soluble drugs could be electrospun into

a water-soluble polymer but such composite fibers are not suitable for in vitro drug

release as they can quickly dissolve in blood or tissue fluid[30]. Thus, the emulsion

electrospinning method was proposed to overcome these limitations. The major steps

of emulsion electrospinning comprises emulsification to form a water/oil (W/O)

emulsion, dissolution of a fiber forming polymer, and electrospinning of the emulsion,

the schematic illustration of emulsion electrospinning was nicely shown by Qi and

co-workers.(Figure 3)[31]. As an example, Li and co-workers have shown that

successful encapsulation of proteinase K as a model protein into poly(ethylene

glycol)-b-poly(L-lactide) (PELA) fibers by emulsion electrospinning comprises a

12  

sustained release of proteinase K after an initial burst release. Furthermore no

decrease in the proteins activity was observed[31]. Jing and co-workers also have

made use of this technique for the encapsulation of doxorubicin hydrochloride (Dox)

in amphiphilic PELA diblock copolymers[32]. Clearly, emulsion electrospinning

offers many advantages. Besides various combinations of hydrophilic and

hydrophobic systems, in particular for drug encapsulation and modification of drug

release, emulsion electrospinning will undoubtedly extend the application of

conventional electrospinning in biomedical fields.

Figure 3. Schematic illustration of emulsion electrospinning (reproduced with

permission from year 2006 American Chemical Society[31]).

Suspension electrospinning

Although emulsion electrospinning provides more possibilities in biomedical

applications, its concept still make use of toxic and/or flammable organic solvents

either as a continuous phase (W/O emulsion) or a separated phase (O/W emulsion)

which could hinder the actual in vivo medical applications and the development for

large scale productions and applications in agriculture. This demands a need for

further solutions regarding electrospinning of polymers only from water which termed

as “green electrospinning”. An alternative to this could be electrospinning of polymer

13  

suspensions from water as continuous phase. Suspension electrospinning comprises

two strategies: electrospinning of primary latex and secondary latex suspensions. The

primary latex suspension can be obtained directly by emulsion or miniemulsion

polymerization which are well established with a large technical scale, but are

available only for a limited number of polymers. Often they require surfactants, which

would be hazardous for biomedical or agricultural applications. In previous work in

our group, successful electrospinning of polystyrene suspensions was achieved by

combination of a small amount of fiber forming water-soluble polymer[33], which

acts as a kind of template polymer as schematically shown in Figure 4[34]. By

removing the water-soluble polymer, “corn-type” polystyrene nanofibers were

obtained, whose stability depends significantly on the size of the latex particles.

Smaller latex particles, form more stable hexagonal arrangement along the fiber main

axis. Although the concept of electrospinning of primary polystyrene particles was

successful, the latex fibers formed were too brittle after removal of the template

polymer and therefore were of no use for any applications. By applying lower Tg latex

particles, inter and intra-latex particle cross-linking was proved a useful method to

improve the mechanical stability[35, 36].

Figure 4. Schematic description of suspension electrospinning primary lattices.

14  

Electrospinning of secondary latex suspensions-water stable nanofibers from

water phase

Electrospinning of water-based secondary lattices could be a new field leading to a

wealth of novel electrospun systems and green electrospinning which is free of

surfactants and harmful organic solvents. One example is the preparation of

amphiphilic biodegradable copolyesters which were processed to high solid content of

secondary suspensions by dialysis. Electrospinning of this system in the presence of a

small amount of water soluble polymer resulted in smooth electrospun fibers. The

whole electrospinning process is free of organic solvents and surfactants which are

paving the way for more promising applications of the feasible method of

electrospinning of useful nanofiber nonwovens, the details about the electrospining of

secondary latex suspensions is described in the main part of this thesis.

1.2.3.3 Applications of electrospun functional nanomaterials

As electrospinning is a remarkably simple and powerful technique for generating 1D

composite nanomaterials, it has received increasing attention during last decades.

Because of the multifunctional properties of the composite materials, they have

already been applied in many areas such as biomedical fields[37, 38], nano-electronic

and optical devices, chemical and biological sensors[39, 40], environment and energy

fields[41]. Here only few of successful applications from biomedical,

stimuli-responsive sensors and energy sector were emphasized.

Biomedical applications

The biomedical field might be one of the most important application areas utilizing

the electrospinning technique. Due to the facile production of very thin fibers of few

nanometers in diameters and therefore with large surface areas, ease of

functionalization and processing, the applications of electrospinning mainly consists

of tissue engineering, drug release, would dressing, enzyme immobilization etc. In the

tissue engineering application, among the essential properties for scaffolds besides

biocompatibility, biodegradability and mechanical properties, the scaffold architecture

15  

is very important for transportation of nutrients and waste during cells proliferation

and differentiation as well as it affects cell binding[42]. Electrospinning provides

possibilities to generate loosely connected three dimensional (3D) mats with high

porosity, interconnected pores and high surface area which can mimic extra cellular

matrix (ECM) structure and thus makes itself an ideal candidate for use in tissue

engineering. Stevens and co-workers[42] demonstrated that the cells binding to

nanoscale architectures, which have bigger surface area, absorb proteins and present

more binding sites to cell membrane receptors, while on microscale architecture, the

cells spread as if cultured on flat surfaces (Figure 5).

Figure 5. Scaffold architecture affects cell binding and spreading(Copyright © 2005,

American Association for the Advancement of Science[42]).

Besides utilized as powerful tools to prepare 3D scaffolds in tissue engineering,

another interesing example to show the potential of electrospun nanoifber in the

biomedical field is the preparation of attolitre-volume reactors. As shown in Figure 6,

Anzenbacher and co-workers utilize 100-300 nm polymer nanofiber as nanocarriers

 

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18  

that various enzymes could be encapsulated in nanoporous silica nanofibers by the

electrospinning method, which act as excellent biosensors.[46]

Environmental and Energy applications

The electrospun nanofiber mats are a potential candidate as an environment-cleaning

material due to its large surface area, porous structure, and cost-effective preparation.

As an example, Ramakrishna and co-workers have fabricated 1D composite

nanomaterias containing poly(vinylchloride) polymer and a catalyst for the

detoxification of nerve agents, which have been prepared from β-cyclodextrin and

o-iodosobenzoic acid.[47]

Figure 8. (A) Electrochemical cell for the simultaneous study of different electrode

materials. (B) SEM images of biofilms in the porous 3D-ECFM after electricity

generation. (Copyright (2011) Royal Society of Chemistry [51])

Besides environmental protection, electrospun nanofibers were also applied in

advanced technology and devices for highly efficient and clean energy generators[48,

49]. Ramakrishna and co-workers reported for the first time a simple method of

fabricating TiO2@CdS nanorods (NRs) by combining electrospinning and successive

ionic layer adsorption and reaction (SILAR) techniques sequentially. The photovoltaic

application was explored by assembling this nanostructure into quantum dots (QD)

sensitized solar cells which gave a best efficiency of over 0.5 %[50]. Chen and

co-workers showed the successful synthesis of three-dimensional carbon fiber

19  

electrodes prepared by electrospinning and solution blowing. The novel electrodes

were shown to be excellent electrode materials for bioelectrochemical systems such as

microbial fuel cells or microbial electrolysis cells. The bioelectrocatalytic anode

current density is shown to reach values of up to 30 A·m-2, which represents the

highest reported values for electroactive microbial biofilms (Figure 8)[51]. All of

these contributions indicate a bright future for searching ideal materials for production

of clean and renewable energy.

20  

1.3 Aim and concept of this work

The fundamental objective of this work is the formulation of new functional

nanostructured materials including stimuli responsive nanocomposites, nanoparticles

and biodegradable nanofibers as well as the investigation of their properties. Special

attention is placed on the preparation of nanocomposites with water absorption/

desorption controlled by photo-irradiation, thermo-responsive and antibacterial

nanodispersions and fast biodegradable (co)polyester nanofibers for potential

biomedical applications.

The concept of this work is the formulation of nanofibers and nanoparticles utilizing

functional polymers, which include photo-responsive superabsorbent polymers,

antibacterial and thermo-responsive quaternized polyurethanes and biodegradable

(co)polyesters. The methods for preparing the functional nanomaterials are mainly

based on electrospinning and heterophase polymerization. The scheme below

describes the sequence and connection between chapters in the whole dissertation.

For stimuli responsive systems, functional nanocomposites were prepared by

combination of photoresponsive superabsorbent particles with hydrophilic elastic

polyurethane nanofibers by the electrospinning technique. The concept was that the

21  

elastic fiber could serve as a substrate to hold the particles and such nanocomposites

were expected to possess good water absorption/desorption capacity controlled by

photo irradiation, good mechanical strength, and therefore have a big potential in

biosensor and drug release applications. Further we utilized cationic segmented block

copolyurethane to form antibacterial and thermo-responsive dispersions. Such

nano-dispersions with upper critical solution temperature (UCST) behavior have a big

potential to be used for drug encapsulation and controlled release for various

therapeutic applications.

As an extension of nanostructured materials in biomedical applications, fast

biodegradable odd-odd polyester and nanofiber of poly (hexamethylene adipate)-PEO

(PHA-b-PEO) were synthesized. The concept of making odd-odd polyester was that

odd number of carbon atoms in the main chain would hinder the molecular chains to

crystallize and lead to a higher degradation rate. While for avoiding the use of

harmful organic solvents during electrospinning, the “green electrospinning” concept

was introduced by preparing high solid contents of amphiphilic PHA-b-PEO

dispersion, and mixing the dispersion with a small amount of water soluble polymer

and processing into nanofibers by electrospinning. Biodegradable fibers were finally

produced by sacrificing water soluble template polymers.

In the final part of this dissertation, an idea of proper combination of fluorinated

polyimides and electrospinning was attempted. The concept was that the large pores

and surface roughness would decrease the dielectric constant of fluorinated

polyimides. Such polyimides nanofibers would have better properties such as high

hydrophobicity and good thermo-oxidative stability and could be of high use as

insulating materials in dielectrics and filter industry.

22  

2. Summary in German (Zusammenfassung) Diese Dissertation hat die Synthese funktioneller, nanostrukturierter Materialien zum

Gegenstand. Dies beinhaltet Stimuli-responsive Nanomatten-Komposite,

Nanopartikel und bioabbaubare Polyesternanofasern, welche neuartige Eigenschaften

wie eine kontrollierte Wasserabsorption bzw. –desorption aufweisen, über eine

schnelle Temperatur-Responsivität verfügen und auf ihren potentiellen Einsatz in

biomedizinischen sowie mikroelektronischen Anwendungen hin untersucht wurden.

Photoresponsive und superabsorbierende Nanomatten-Komposite wurden durch

Kombination hydrophiler Polyurethan-Nanofasern mit vernetzten, photoresponsiven

und superabsorbierenden Partikeln hergestellt. Die Eigenschaften der Nanokomposite

wiesen dabei eine hohe Abhängigkeit vom Anteil der enthaltenen photochromen

Superabsorberpartikel auf. Die Komposit-Nanomatten verfügen über eine

Wasseraufnahmekapazität von 40 g/g und erreichen ihr Absorptionsmaximum bereits

innerhalb von 2 Minuten, was sie gegenüber konventionellen Superabsorbern

auszeichnet, die dazu üblicherweise länger benötigen. Das elastische Polyurethan

dient in diesem Fall als Matrix zur Fixierung der Partikel und verleiht dem

Nanokomposit darüber hinaus gute mechanische Eigenschaften. Dieses

Komposit-Material bietet sich zum Einsatz im Rahmen einer kontrollierten

Wirkstofffreisetzung an oder auch zur Anwendung im Bereich der Sensorik. Als

weiteres Stimuli-responsives System wurden antibakterielle, stabile, kationische,

segmentierte Urethan-Blockcopolymer-Nanopartikel mit einer oberen kritischen

Lösungstemperatur (upper critical solution temperature, UCST) durch eine

zweistufige Polykondensation von 2,4-Toluoldiisocyanat, Diethanol-N-methylamin

und Polyethylenglycol (PEG) hergestellt. Aus dem Polymer wurde durch einfaches

Erhitzen auf 90 °C und anschließendes Abkühlen auf Raumtemperatur eine stabile

Dispersion erhalten. Die Einführung von PEG-Segmenten wirkte sich positiv auf die

Ausbildung der Dispersion aus ohne dabei die antibakterielle Aktivität einzubüßen;

zudem lassen sich Partikelgröße sowie UCST durch den PEG-Gehalt und die

Konzentration der Dispersion einstellen. Auf diesem Weg wurde eine neuartige,

23  

antibakterielle sowie thermoresponsive Dispersion erhalten, welche ein großes

Potential zur Anwendung im Bereich der Wirkstoffverkapselung und kontrollierten

Freisetzung zu verschiedenen therapeutischen Zwecken birgt.

Der Arbeit an funktionellen, Stimuli-responsiven, nanostrukturierten Materialien folgt

die Einführung von Bioabbaubarkeit in den Kapiteln 3 und 4 als weiterer

Funktionalität. Kapitel 3 behandelt die Synthese schnell abbaubarer, zweifach

ungeradzahliger (odd-odd) Polyester sowie deren Abbauverhalten mit und ohne

Einsatz von Enzymen. Die odd-odd-Struktur der Kettenrückgrats führt dazu, dass der

5,7-Polyester eine relativ geringe Kristallinität aufweist und dadurch im Vergleich zu

kommerziellem Poly-ε-caprolacton (PCL) über eine schnellere Abbaurate verfügt.

Durch Rasterelektonenmikroskopie sowie optische Polarisationsmikroskopie wurde

ermittelt, dass der Abbau dabei sowohl in den amorphen Domänen als auch an der

Oberfläche beginnt, was mit einer Veränderung der Oberflächenmorphologie

einhergeht. Damit wurde der Klasse der bioabbaubaren Polyester ein weiterer

Vertreter hinzugefügt, der über ein eigenes Profil zum Einsatz in mannigfaltigen

biomedizinischen Anwendungen verfügt. Basierend darauf wurden

Polyhexamethylenadipat-Polyethylenoxid (PHA-b-PEO) Blockcopolymere

synthetisiert und zu wässrigen Suspensionen von hohem Feststoffgehalt

weiterverarbeitet. Diese Suspensionen wurden mit einem geringen Anteil an

hochmolekularem Polyethylenoxid versetzt und zu den entsprechenden Nanofasern

elektroversponnen; nach Extraktion mit Wasser wurden stabile PHA-b-PEO

Nanofasern erhalten. Dieses Konzept des Elektrospinnens von Biopolymeren aus

wässrigen Suspensionen mit Verzicht auf gesundheitsschädliche, organische

Lösungsmittel wird als „Grünes Elektrospinnen“ (green electrospinning) nahe gelegt

und bietet neuartige Perspektiven für Anwendungen in den Bereichen Medizin,

Pharmazie und Landwirtschaft.

Der letzte Teil dieser Dissertation beschäftig sich damit, die Charakteristika

elektrogesponnener Nanofasern, wie zum Beispiel ein hohes Oberfläche zu

24  

Volumen-Verhältnis, miteinander verbundene Poren und eine raue

Oberflächenstruktur, mit denen fluorierter Polyimide zu verknüpfen. Dazu wurden

diese durch Elektrospinnen zu Nanomatten verarbeitet, welche über eine niedrige

Dielektrizitätskonstante, eine hohe thermo-oxidative Stabilität sowie Hydrophobizität

verfügen. Neben ihrer Eignung für die Filter- und Kompositindustrie könnten diese

von hohem Nutzen als Isolator in Verbundzwischenschicht-Dielektrika sein.

25  

3. Summary In this dissertation, the synthesis of functional nanostructured materials including

stimuli responsive nanomat composites, nanoparticles and biodegradable polyester

nanofibers are presented. Further the novel properties such as controlled water

absorption/desorption, fast thermo responsive properties and potential applications in

biomedical and microelectronic fields were investigated.

In chapter 4.1, photoresponsive superabsorbent nanomat composites were prepared by

combination of hydrophilic polyurethane nanofibers with crosslinked photoresponsive

superabsorbent particles, the properties of nanocomposites were highly dependent upon

the amount of the superabsorbent photochromic particles added. The composite

nanomats had a high water absorption capacity of 40 g/g and reached to the maximum

absorption in two minutes which was faster than the conventional superabsorbers. The

elastic polyurethane served as a substrate to capture most of the particles and provided

good mechanical properties for the nanocomposties. Such nanocomposites could be of

utility for drug release and sensor applications. Also stimulus responsive antibacterial

cationic segmented block copolyurethane nanoparticles with upper critical solution

temperature (UCST) behavior was synthesized in chapter 4.2 by polyaddition of

2,4-toluene diisocyanate, diethanol-N-methylamine and polyethylene glycol (PEG) in

two steps. Stable dispersions were prepared by facile heating up to 90 °C and cooled

down to room temperature. The introduction of PEG segments was found to favor the

formation of stable dispersion and keep the antibacterial activity, the particle size and

UCST could be adjusted by the PEG contents and concentration of the dispersions.

Such novel antibacterial dispersion had a big potential to be used for drug

encapsulation and controlled release for various therapeutic application.

Followed the previous research on functional stimuli responsive nanostructured

materials, biodegradability functionality was introduced and investigated in the

following two chapters. Chapter 4.3 describes a fast degrading odd-odd aliphatic

polyester synthesis and degradation behavior with and without enzyme. Due to the

26  

odd-odd structure of the main chain, the polyester-5,7 had a relative low crystallinity

and possessed a faster degradation rate compared to commercial poly(ε-caprolactone)

(PCL). The degradation was started in the amorphous region and on the surface with

change in surface morphology which was confirmed by SEM and optical polarized

microscopy. It would be an addition to the class of biodegradable aliphatic polyesters

having its own profile for different biomedical applications. Based on this study,

poly(hexamethylene adipate)-PEO (PHA-b-PEO) block copolymers were synthesized

and processed to aqueous suspensions with high solid contents. This suspension was

mixed with a small amount of high molecular weight PEO and electrospun into

corresponding nanofibers. The stable nanofibers of PHA-b-PEO were obtained after

extraction by water. Such concepts of utilizing electrospinning of biopolymers from

aqueous suspensions avoiding harmful organic solvents are suggested to be “green

electrospinning” and offer novel perspectives for application in actual medicine,

pharmacy and agriculture.

In the last part of this dissertation, utilizing the characteristics of electrospun

nanofibers (e.g. high surface to volume ratio and rough surface structures), fluorinated

polyimides were processed into nanomats by electrospinning techniques. The

corresponding nanofibers have a low dielectric constant, high thermo-oxidative

stability and hydrophobicity which could be of high use as insulating material in

interlayer dielectrics besides their use in filter and composite industry.

27  

4. Cumulative part of dissertation

4.1 Stimuli-Responsive Elastic Polyurethane-Based Superabsorber Nanomat

Composites

4.1.1 Summary and discussion

This work shows the success of making photoresponsive superabsorber nanomats.

They are made by the combination of a porous hydrophilic polyurethane nanofiber

matrix with photoresponsive superabsorbent particles. The resulting nanocomposites

have very high loading (up to 50 wt%), good water absorption capacity (4000 %) and

relatively good tensile strength (3 MPa). Such nanocomposites could be of utility not

only for house hold cleaning purposes but could be applied in drug release and

sensors.

The trans-to-cis isomerization is well known to change the distance between the 4 and

4’ carbons of the aromatic rings and thereby causes a macroscopic volume change in

the polymer on photo irradiation. In this work, the photoresponsive superabsorber

particles containing a crosslinked hydrophilic core and a hydrophobic azobenzene

containing shell was synthesized by the known procedure. [Soft Matter, 2008, 4,

768-774]. The chemical structure of the particles and illustration of water removal

from swollen photoresponsive absorbent polymer by light irradiation is given in

Schemes 1 and 2. This novel superabsorbent and photochromic particles were

prepared via inverse suspension polymerization, the hydrophilic crosslinker

N,N’-methylene bisacrylamide (BIS) and photochromic crosslinker

The manuscript about the content of this chapter has already been published.

Fei Chen, Andreas Greiner, Seema Agarwal*, Stimuli Responsive Elastic Polyurethane based Superabsorber Nanomat Composites, Macromolecular Materials and Engineering, 2011, 296, 517–523

 

bis(

and

O

HN

Schmak

Schligh

The

onto

nan

elas

with

part

(methacrylo

shell of the

NH

ON

ONH2

heme 1. Cheking superab

heme 2. Waht irradiation

e synthesize

o the hydro

ofiber serv

sticity to th

h 50 wt% o

ticles were c

oylamino)az

e particles.

COOH COO-Na+

N

N

emical strucbsorber nan

ater removan.

ed particles

ophilic TPU

ed as a sub

e hydrogel

of the PSAP

captured an

zobenzene (

+

NH

NN

O

NH

O330-3

> 400nm

cture of the nocomposite

al of swoll

have diame

U nanofiber

bstrate for h

particles. T

Ps was dep

nd stabilized28 

(AC) was u

NO

HN

380nm

or heat

photorespoes.

len photore

eters of arou

rs by the p

holding the

The typical

picted in Fig

d by the elas

used respect

NH

O

ONH2

CO

onsive super

sponsive su

und 50-100

process of

e particles a

l morpholog

gure 1. It w

stic TPU na

tively to ge

OOH COO-Na+

NH

N

rabsorber pa

uperabsorbe

μm and we

electrospinn

and provide

gy of comp

was clearly

anofibers. Th

enerate the

N

ONH

articles used

ent polyme

ere immobil

ning. The T

ed strength

posite nanom

shown that

his morpho

core

O

d for

r by

lized

TPU

and

mats

t the

logy

29  

of particles trapped between the nanofibers could be one of the most important

reasons account for avoiding “gel effect”, a disadvantage in the conventional hydrogel

particles.

Figure 1. SEM pictures of (A) PSAP particles and (B) 50 wt% PSAP particles in TPU nanofibers.

The absorbency tests of nanomats were performed using water. Figure 2(A) shows a

graph of the equilibrium absorbency of water containing 0-50 wt% PSAP particles at

room temperature. The absorbency capacity increased with an increase in the amount

of particles in the composite. The maximum absorbency capacity of water was 45 g/g

for composites with 50 wt% particles. Also, the composite superabsorbent nanomat

had a much faster absorbency rate (60 s) to reach to equilibrium compared to the

normal absorbent hydrogel particles (at least 5-10 min). Figure 2 shows the

exceptional elastic properties of these nanocomposites.

(A)  (B) 

30  

Figure 2. Rate of absorption of composite nanofiber with different contents of

absorbent PSAP particles at room temperature (left) and pictures to show the good

elastic properties of nanocomposites (right).

For studying the photoresponsivity of the composite nanomats, the weight loss of the

equilibrated composites with water on irradiation at 350 nm was determined at 30 min

time intervals (Figure 3). Two samples were prepared for the weight loss test. The

results showed that TPU/PSAPs composite fibers showed higher weight loss than the

sample without irradiation.

0 1 2 3 4 5 60.0

0.2

0.4

0.6

0.8

1.0 PSAP NF irr PSAP NF

Nor

mal

ized

wei

ght l

oss(

g)

Time(h)

Figure 3. Weight loss of TPU/PSAP composite nanofiber with 32 % of PSAPs particles on irradiation. “irr” means irradiation.

The details of this work are published in Macromolecular Materials and Engineering, 2011, 296, 517–523 and attached in Appendix 8.1.

31  

4.1.2 Declaration of my contribution

The preparation of composite polymeric nanomats and characterization was carried

out by me. The draft of the manuscript was written by me. Prof. Dr. Andreas Greiner

proposed many useful suggestions for this project. Prof. Dr. Seema Agarwal gave the

total support and main correction for the manuscript.

32  

4.2 Multifunctional Polyurethane Aqueous Dispersions showing Thermo

Responsivity with UCST and Antibacterial Properties

4.2.1 Summary and discussion

In the chapter 4.1, superabsorbent and photo-responsive particles were synthesized

and combined with nanofibers to form nanomat composites. These functional

nanocomposites have a big potential for house hold cleaning, drug release and sensor

applications. For household cleaning applications, besides absorbency and desorbing

capability, a growing public awareness of hygiene and the pathogenic effects, stain

and malodor formation resulting from microbial contamination has caused a quickly

increasing use of biocides in personal care. Therefore, in this work, with an aim to

synthesize an antibacterial stable dispersion with facile way for personal care,

segmented block co-polyurethane nanoparticles with upper critical solution

temperature (UCST) behavior are studied.

Polymers with quaternary ammonium groups are a kind of important antibacterial

polymers. Their mechanism of antibacterial action is primarily the targeting and

disruption of the bacterial cell membrane. Due to this very general mode of action,

these compounds can be used to destroy a wide range of bacteria and the activity can

potentially be recovered by removal of dead cell material from the polymer surface.

Firstly, the segmented block copolyurethane base polymer P(TDI-DEMA-PEG) was

synthesized by a standard polyaddition procedure using 2,4-toluenediisocyanate (TDI),

diethanol-N-methylamine (DEMA) and PEG (Mn 2000) as monomers according to

Scheme 3. The chemical structure was confirmed by nuclear magnetic resonance

The manuscript about the content of this chapter was already submitted.

Fei Chen, Judith Hehl, Yu Su, Claudia Mattheis, Seema Agarwal*, Multifunctional Polyurethane Aqueous Dispersions showing Thermo Responsivity with UCST and Antibacterial Properties, Journal of Colloid and Interface Science. 2011, Submitted.

33  

(NMR) spectroscopy, an obvious peak at 3.51 ppm which is assigned to the CH2

group from PEG confirmed that PEG was successfully introduced into the copolymer.

By calculating the integration of signals from CH2 groups of PEG (3.51 ppm) with

-NCH3 groups (2.3 ppm) from DEMA, it was found that the composition of PEG in

copolymers fits very well with the feeding ratio; furthermore no signs of urea linkages,

but only urethane linkages were observed in the copolymers (data shown in appendix

8.2). In the following discussion, base homo-polyurethane and quaternized

polyurethane are denoted as PU-0 and QPU-0, co-polyurethane and quaternized

co-polyurethane are named as PU-X and QPU-X, with X referring to the molar ratio

of PEG2000 in the copolymers.

The procedure to prepare the dispersions is very simple; the polymers were directly

dissolved in hot water (98 °C), after cooling to room temperature, opaque dispersions

were formed which showed obvious UCST behavior. The dispersion stability and

particle size analysis revealed that by incorporation of PEG segments into the

copolymers, the solid content of the dispersion can be increased but the stability did

not decrease accordingly. The results are summarized in Table 1.

OCN NCO HON

OHOOHH n

OO

HN

HN

O

ONO

O

HN

HN

O

On p q

PEG2000 TDI DEMATHFDABCO

RXDMF

OO

HN

HN

O

ONO

O

HN

HN

O

On p qR

X

Scheme 3. Synthetic route of the base polymer PU-0 and quaternized QPU-0 from

2,4-toluene diisocyanate (TDI) and diethanol-N-methylamine (DEMA).

34  

Table 1. Dispersion experiments of the cationic polyurethanes.

+: soluble -: insoluble *: dispersion & gel like precipitate

QPU-X C(PU)

/% wt/wt

Solubility Dispersion

QPU-0 ≤5

10

+

-

*

QPU-1 >1≤8 + *

QPU-3 >1≤10 + *

QPU-5 >1≤12 + *a

QPU-10 >1≤20 + solution

a: quaternized co-polyurethane(QPU-5) polymers can form a dispersion only above

5wt%.

Particle sizes were determined by DLS for different concentrations of the aqueous

dispersion of QPU-1. A summary of the results for all measurements is shown in

Table 2. An obvious increasing particle size was observed with increasing dispersion

concentration, the particle size increased from 70 nm at 1 wt% to around 1000 nm at 8

wt% for QPU-1. The particle size measurement by DLS was also done for the same

concentration of copolyurethane with different PEG ratios. There is no big difference

of particle size among all the samples, the only change can be observed is that by

increasing the PEG content in the copolymer, the particle size distribution decreased

but the particle size kept constant around 80 nm.

35  

Table 2. Particle size of quaternized PU dispersions.

QPU Diameter

/nm

QPU Diameter

/nm

1% QPU-0 128±97 1% QPU-1 71±26

1% QPU-1 71±26 2% QPU-1 167±53

1% QPU-3 100±35 3% QPU-1 369±164

1% QPU-5 136±58 4% QPU-1 358±94

5% QPU-1 563±28

6% QPU-1 1054±122

8% QPU-1 863±93

The UCST behavior of the QPU dispersions was investigated using turbidity

photometric measurements. The quaternized polyurethane dispersions with the same

concentration but with different contents of PEG in the composition were studied. It

was expected that the UCST would decrease with increasing content of PEG in the

copolymers, but the results were reverse with our expectation. Figure 4 shows the

UCST increased with increasing PEG content in the first stage but later decreased

again when the PEG content reached to 5 mol%. Surprisingly the 5 wt% dispersion of

QPU-1 also showed LCST behavior around 37.7°C (data not shown here). This

complex UCST phenomenon was probably due to the contribution of PEG segments

and further investigation needs to be done to fully understand it.

36  

70 72 74 76 78 80 82 84 86 88 90

0

20

40

60

80

100

120

Tran

smitt

ance

(%)

Temperature(°C)

QPU-0 QPU-1 QPU-3 QPU-5

Figure 4. Turbidity measurements of 5 wt% quaternized PU with different PEG

contents to investigate the influence of PEG on the UCST behavior of copolymers

(first cooling cycle).

The antibacterial properties of the dispersions were investigated by several standard

methods. All dispersions proved to be active against E. coli; the determined MIC and

MBC values do not differ among the tested dispersions (see Table 3) and are with

concentrations of 78 µg/ml as MIC (minimum inhibition concentration) and

156 µg/ml as MBC (minimum bactericidal concentration) in a good range. When the

focus comes to the speed of antibacterial action, the homo cationic polyurethane

dispersion showed to be the best. Already after 10 minutes of contact, the dilution

with a concentration of 2.5 mg/ml and all higher concentrated samples killed > 99.9%

of the cells, as shown in Figure 5. After a longer contact time of 120 minutes also the

other dispersions showed a total reduction of bacteria growth at concentrations of

≥ 625 µg/ml or even ≥ 313 µg/ml as depicted in Figure 5.

37  

Table 3. Minimum inhibition concentration (MIC) and minimum bactericidal concentration (MBC) of the dispersions towards a 105 cfu/ml suspension of E. coli.

Sample MIC / µg ml-1 MBC / µg ml-1

QPU-0 78 156

QPU-1 78 156

QPU-3 78 156

QPU-5 78 156

Figure 5. Reduction of bacteria growth for different dilution stages of the dispersions after (A) 10 minutes and (B) 120 minutes contact to E. coli (105 cfu/ml).

The details of this work was already submitted to Journal of Colloid and Interface Science and attached in Appendix 8.2.

38  

4.2.2 Declaration of my contribution

The plan and execution of synthesis of co-polyurethane and the manuscript was done

by me. Yu Su made research practical with me on this topic and helped in synthesis

and characterization. Claudia Mattheis performed the antibacterial tests. Prof. Dr.

Seema Agarwal gave the total support and main correction for the manuscript.

39  

4.3 A Fast Degrading Odd-Odd Aliphatic Polyester-5,7 made by Condensation

Polymerization for Biomedical Applications

4.3.1 Summary and discussion

Previous two chapters have shown the preparation and characterization of functional

stimuli-responsive nanostructured materials. For different applications includes tissue

engineering and drug release, biodegradability functionality is of great importance.

Aliphatic polyesters received continued attention as it is an important kind of

environmental friendly and degradable polymers for medical and non-medical

applications. The degradability rate depends upon the type of chemical linkage,

molecular weight, hydrophobicity, crystallinity, flexibility etc. Among all these

parameters crystallinity has a key influence on biodegradation behavior, thus in this

work a novel polyester with odd carbon atoms in the main chain was synthesized

starting from pentanediol and 1,7-heptanedioicacid by polycondensation reaction in

the presence of titanium tetrabutoxide (TBT) as catalyst. The reaction was carried out

in two steps and polyphopsphoric acid was used as heat stabilizer (Scheme 4).

The structural characterization of the resulting polymer was confirmed by 1D and 2D

NMR spectroscopy. The molecular weight, thermal stability etc. were investigated by

using gel permeation chromatography (GPC), differential scanning calorimetry (DSC)

and wide angle X-ray diffraction (WAXD). A single melting peak was seen at about

43 °C but no clear glass transition could be seen in the DSC. In an attempt to observe

the glass transition temperature, a new tool for determining the glass transition

The manuscript about the content of this chapter was already accepted.

Fei Chen, Jan Martin Nölle, Steffen Wietzke, Marco Reuter, Sangam Chatterjee, Martin Koch, Seema Agarwal*, A Fast Degrading Odd-Odd Aliphatic Polyester-5,7 made by Condensation Polymerization for Biomedical Applications, Journal of Biomaterials Science: Polymer Edition. 2011, Accepted.

40  

temperature was employed: non-contact, non-destructive terahertz time-domain

spectroscopy (THz TDS).

Scheme 4. Synthetic scheme for the formation of polyester-5,7 by polycondensation

of 1,7-heptanedioic acid and 1,5-pentanediol.

Temperature-dependent THz TDS measurements reveal the glass transition by a

change in the thermal gradient of the THz refractive index. This step marks the

beginning of the translational motion of backbone chain segments in the amorphous

domains. Due to the model of the free volume, there is a decrease in density with

increasing temperature that is higher at temperatures above the glass transition than

below. This two regime behavior is reflected by the refractive index according to the

Lorentz-Lorenz law. Figure 6 depicts the temperature-dependent refractive index of

the biodegradable polyester-5,7 at 1.0 THz (33 cm-1). Both temperature regimes can

be fitted by a linear regression. The intersection yields the glass transition temperature

Tg in the vicinity of -53 °C.

Figure 6. THz refractometry reveals the glass transition temperature at the

intersection of two linear fits representing the two temperature regimes of the

refractive index at 1.0 THz (33 cm-1).

HO

O

OH

O

HO OH

HO

O

O

O

On

H

TBT, PPA,190-230 °C

1 bar...0,2 mbar

41  

The enzymatic degradability of the polyester-5,7 was evaluated using lipase from P.

Cepacia at 37 °C. A comparison of degradation rate was done with commercial

standard polycaprolactone (PCL) with similar chemical structure in the main chain.

The weight loss profile of polyester-5,7 and PCL at different time intervals in

presence of the lipase enzyme is shown in the Figure 7. A very fast degradation of

polyester-5,7 can be seen from the curve in comparison to PCL; after 8 h in 0.2 mg/ml

pseudomonas lipase buffer solution, the weight loss was already about 53 %, after 22

h more than 90 wt% of the samples degraded to water soluble products, which were

further analyzed by 1H NMR and GPC. The degradation products of polyester in

water after 8 h of degradation was monitored by 1H NMR spectroscopy, the results

showed the degradation products mainly consist of 1,5-pentanediol, 1,7-heptanoic

acid and oligomers (data not shown here).

0 10 20 30 40 50 60 70 80 90

0

20

40

60

80

100

120

Wei

ght l

oss

(wt.

%)

Degradation time (hour)

Polyester-7-5 PCL

Figure 7. Weight loss profiles of polyester-5, 7 and PCL; degradation in phosphate

buffer (pH 7.0) in presence of Pseudomonas lipase (0.2mg/ml).

To further understand the reason of the fast enzymatic degradation of polyester-5,7,

optical polarized microscopy and SEM were employed to investigate the crystalline

spherulites and surface morphology during degradation. Polyester-5,7 before

degradation was semicrystalline with about 50 % crystallinity, calculated based on the

heat of fusion observed in the first heating cycle from DSC (data not shown here), and

the bigger spherulites as observed by optical polarized microscope. The percentual

42  

crystallinity calculated for PCL was about 60% (melting enthalpy of 136 J/g for 100%

crystalline PCL is taken from the literature) and smaller spherulites formed (Figure 8).

After degradation for about 8h, the percentual crystallinity increased in the left over

samples of polyester-5,7 and PCL to about 67-68% but with smaller spherulites for

PCL. This could be due to the degradation in the amorphous region thereby increasing

the relative crystallinity of the degraded sample.

Figure 8. Optical microscope pictures of polyester-5,7 (A) before degradation (B)

after degradation (8 h) (C) polycaprolactone (PCL) before degradation and (D) PCL

after degradation (8 h).

SEM was used to investigate the surface morphological changes during degradation.

The scanning electron micrographs of polyester before and after degradation are

shown in the Figure 9. Degradation led to surface erosion; fibrillar and sponge like

structures were observed. The SEM characterization of the cross section of the

polyester film before and after 22 h degradation in lipase was also done. The

(A)  (B)

(C)  (D)

43  

morphology after 22 h degradation remained the same as before degradation, which

showed degradation was probably due to surface erosion. The degradation behavior

observed for polyesters was almost the same as observed for the known degradable

polycaprolactone except its fast rate of degradation.

Figure 9. SEM pictures of polyester-5,7 and PCL films during degradation; (A) Polyester-5,7 before degradation (B) after 22 h of degradation; (C) PCL before degradation (D) after 22 h of degradation.

The details of this work was already accepted by Journal of Biomaterials Science: Polymer Edition and attached in Appendix 8.3.

(B)(A) 

(C)  (D)

44  

4.3.2 Declaration of my contribution

The synthesis work of polyester-5,7 was done by Jan Martin Nölle. The structural

characterization, enzymatic degradation and all other tests were carried out by me.

The Terahertz time-domain spectroscopy (THz TDS) was done by the research group

of Prof. Martin Koch (group members of Steffen Wietzke, Marco Reuter and Sangam

Chatterjee). The draft of manuscript was written by me and Prof. Dr. Seema Agarwal

gave the total support and main correction for the manuscript.

45  

4.4 Nanofibers by Green Electrospinning of Aqueous Suspensions of

Biodegradable BlockCopolyesters for Applications in Medicine, Pharmacy and

Agriculture

4.4.1 Summary and discussion

In Chapter 4.3, a fast enzymatic degradable polyester-5,7 with odd carbon atoms in

the main chain was successfully synthesized by polycondensation in melt. This novel

polyester could be processed into nanofibers by electrospinning and utilized in

regenerative scaffolds for tissue engineering. Like all other polyesters, these polymers

have the property in common only to dissolve in harmful organic solvents, thereby

limiting their utility for actual biomedical and agricultural applications. One feasible

method to solve this limitation is to make aqueous suspensions by solvent

displacement methods or emulsions etc. from these biodegradable polyesters. With a

small amount of matrix polymer, like high molecular weight PEO, they can be

processed into stable nanofibers by extraction of the template polymer. This novel

concept to prepare biodegradable nanofibers avoiding organic solvent is claimed here

as “green electrospinning” (Figure 10). This requires the development of a high solid

content polyester suspension in water where the melting point of the polyester blocks

should be relatively low.

Generally the secondary suspensions of homo-polyesters have a very low solid

content due to its high hydrophobicity in water. Thus in this chapter, with the aim to

develop a high solid content of polyester dispersion in water, biodegradable

poly(hexylene adipate)-block-(methoxypolyethylene oxide) copolymers (PHA-PEO)

The manuscript about the content of this chapter has already been published.

Jinyuan Sun, Kathrin Bubel, Fei Chen, Thomas Kissel, Seema Agarwal, Andreas Greiner*, Nanofibers by Green Electrospinning of Aqueous Suspensions of Biodegradable Block Copolyesters for Applications in Medicine, Pharmacy and Agriculture, Macromol. Rapid Commun. 2010, 31, 2077–2083.

46  

were synthesized and processed to high solid content dispersions. This suspension

next was mixed with a small amount of high molecular weight PEO as well as

polyoxyethylene-20-stearyl ether  (Brij78) and electrospun into the corresponding

nanofibers. After extraction with water, nanofibers of PHA-PEO were obtained.

Scheme 5. Synthesis of poly(hexylene adipate)-block-(methoxypolyethylene oxide) (PHA-b-PEO)

The synthesis of the diblock copolyesters was performed by melt polycondensation,

as shown in Scheme 5. The expected chemical structure of PHA-b-PEO was

confirmed by 1H NMR, 13C NMR and IR spectroscopy. (Details can be seen in the

appendix publication 8.4).

PHA-b-PEO was dissolved in acetone, after addition of a solution of water and Brij78,

ultrasound was applied. Finally, acetone was removed by slow evaporation at room

temperature and milky aqueous suspensions without any precipitation were obtained

with contents of 2.94 wt%. Analysis of the particle diameter by dynamic light

scattering showed average particle diameters of 177 nm. Although the suspensions

were stable at room temperature for at least 10 d, the suspensions were of too low a

solid content to be useful for nanofibers preparation by suspension electrospinning.

For achieving higher concentrated suspensions, dialysis method with PVA (15 wt% in

water) was selected as outer media and 2.5 wt% aqueous suspension with Brij 78 was

kept in dialysis membrane(MWCO 10 000). Due to the osmotic pressure difference, a

constant weight loss of the original suspension in the dialysis tubes was measured

with 100 h, resulting in a corresponding aqueous suspension of 16 wt% PHA-b-PEO.

47  

This milky suspension did not show visible precipitation and displayed an average

particle diameter of 108 nm by dynamic light scattering.

Figure 10. The schematic diagram of “green electrospinning” to form biodegradable nanofibers.

As mentioned previously, water soluble polymers are required as template polymers

for “green electrospinning”. Therefore, electrospinning of PHA-b-PEO was

completed with different amounts of PEO relative to PHA-b-PEO. After

electrospinning, the composite fibers were treated with water in order to test the water

stability of the fibers. Fiber diameters of the as spun fibers ranged from 350 to 550 nm

(Figure 11). After treatment with water at 20 °C for 24 h, the fibers showed no sign

swelling or disintegration. The fiber surfaces of the as spun fibers appeared to be

somewhat smoother that the corresponding fibers after water treatment.

As spun fibers and water treated fibers were also analyzed with 1H NMR

spectroscopy (Data no shown here), no traces of PEO and Brij78 were found in the

water treated fibers, thereby showing the complete removal of PEO and the surfactant.

This also proved the success of the concept of making stable biodegradable nanofiber

non-wovens from secondary water suspensions free of harmful organic solvents.

In sum, biodegradable PHA-b-PEO diblock copolyesters were successfully

synthesized and processed by dialysis method to aqueous secondary suspensions of 16

wt%. With the template polymer PEO, water stable biodegradable PHA-b-PEO

48  

nanofiber non-wovens were formed by extraction of PEO. The method developed in

this chapter will open up many perspectives for water based electrospinning and

expand the biodegradable nanofibers in actual biomedical and agricultural

applications.

Figure 11. SEM of electrospun fibers of PHA-PEO-2b with 4% PEO, (A) before and (B) after water treatment for 2 d at 25 °C at higher magnification.

The details of this work have already been published in Macromol. Rapid Commun. 2010, 31, 2077–2083 and attached in Appendix 8.4.

(A)  (B)

49  

4.4.2 Declaration of my contribution

The synthesis of PHA-b-PEO and secondary dispersion was done by Jinyuan Sun.

The initial electrospinning of dispersions with PEO template and SEM

characterization was done by me, the chemical structure confirmation before and after

water treatment for the as spun fiber was done by Kathrin Bubel. Prof. Dr. Andreas

Greiner gave the total support and main writing the manuscript and Prof. Dr. Seema

Agarwal contributed for the correction of the manuscript and useful discussion.

50  

4.5 Low Dielectric Constant Polyimide Nanomats by Electrospinning

4.5.1 Summary and discussion

The previous four chapters described the potential application of responsive and

biodegradable nanofibers in house hold cleaning, medicine, pharmacy and agriculture.

The assumption of this chapter was the combination of proper materials (fluorinated

polyimides) with the electrospinning technique could produce low dielectric constant

materials used as insulating material in interlayer dielectrics due to the large pores in

electrospun nanofibers and the high surface to volume ratio.

With an aim to provide low dielectric constant polyimide membranes with good

processability, high thermal stability and low water absorption, we choose fluorinated

polyimides and use electrospinning to form nanomats. The polyimides in this work

were based on 4,4-Bis[3'-trifluoromethyl-4'-(4'-amino benzoxy)benzyl] biphenyl (Q)

and virous fluorinated and non-fluorinated dianhydrides. The detailed characterization

in terms of thermal stability, dielectric constant and hydrophobicity was also reported.

The synthesis of poly(ether imides) were carried out by two steps via poly(amic acid)

intermediate (scheme 6), the highly viscous poly(amic acid) solutions were diluted to

different solid contents and electrospun (Figure 12). The nanofibers were collected on

a rotating drum shaped roller with high rotating speed to form aligned nanofiber belts.

The fiber morphology and dimensions of the nanofibers mats was investigated using

SEM. It was observed that the nanofibers were aligned longitudinally and the average

diameter varies from 100-200 nm.

The manuscript about the content of this chapter has already been published.

Fei Chen, Debaditya Bera, Susanta Banerjee*, Seema Agarwal*, Low Dielectric Constant Polyimide Nanomats by Electrospinning, Polymers for Advanced Technologies, 2011, early view online.

51  

O OH2N NH2

CF3

F3C

+ O

O

O

O

O

O

Ar

DMF / 20 0C

Polyamic acid

Thermal Imidization

O O

CF3

F3C

O

O

N

O

O

Ar

n

Ar =

O F3C CF3

PMDA BPDA BTDA 6-FDA

Scheme 6. Reaction scheme for the synthesis of poly(ether imide)s.

Figure 12. (A) Schematic illustration of electrospinning setup for collecting the aligned PPA nanofiber and (B) SEM images of aligned Q-6FDA-i nanofiber.

Further, electrospun polyimides mats were evaluated for their mechanical properties

(Figure 13). The polyimide mat prepared from BPDA showed the highest tensile

strength at break. The value is much higher than the polyimide film prepared from

BPDA. The results also showed the importance of rod-like structure of polyimide in

getting thigh strength and high modulus fibers as Q-FDA and Q-BTDA nanofiber

52  

mats have relatively lower mechanical strength compared to their correspondent

films.

Figure 13. Stress-strain curves of (A) polyimide mats and (B) films.

The dielectric constant of the polyimide mats and the films were calculated from their

corresponding capacitance values at 1 MHz. The dielectric constant values are

summarized in Table 4. It was observed that the dielectric constant of the aligned

nanofiber mats were considerably lower than their analogous film samples. The

values are much lower than the commercially available polyetherimide ULTEM 1000

and polyimide Kapton H at 1 kHz. The lower dielectric constant of these polyimide

nanofiber mats was probably due to the large pores and high surface to volume ratio

between the nanofibers.

For microelectrics industry, low moisture absorption and hydrophobic surface is also

a key consideration, thus the hydrophobic behavior of polyimide films and polyimide

nanomats were evaluated by water contact angle test. Figure 14 showed all polyimide

mats have hydrophobic nature with highest value of contact angle about 125° for

fluorinated polyimide mat Q-6 FDA-i. Another obvious observation is that all the

electrospun mats have higher contact angles compared to the corresponding films, this

can be attributed the porous rough nano structures surface generated by

electrospinning enhances the hydrophobicity.

B A 

 

Tabfibe

Sa

Q-P

Q-B

Q-B

Q-6

Figu(con

In s

fluo

biph

dian

bis-

con

100

stab

ble 4. Wateer mats and

ample

PMDA-i

BPDA-i

BTDA-i

6FDA-i

ure 14. Difntact angle

sum, polyim

orinated diam

henyl (Q) w

nhydrides

-trifluorome

stants-lowe

00 and PI

bility and en

er contact athe polyimi

Aligned p

Contac

angle(°

114±5

118±4

120±3

125±4

fferent wett125±4o) (B)

mide nanofib

mine mono

with differe

in two s

ethyl grou

er than thos

Kapton H

nhanced hy

angle and dide films.

polyimide m

ct

°)

Diecoat

5

4

3

4

ting behavi) Q-6FDA-i

ber mats an

omer 4,4-Bi

ent commer

steps. The

ups (Q-6F

e of the co

at 1 kHz-

ydrophobici

53 

dielectric co

mats

electric onstant 1 MHz

1.739

1.607

1.765

1.430

iors of (A) i film (conta

nd films hav

s[3'-trifluor

rcially avai

e amorpho

FDA) exh

mmercially

-but also ex

ity relative

onstant of t

Poly

Conta

angle(

84±5

77±2

75±2

83±3

polyimide act angle 83

ve been pre

romethyl-4'-

lable fluori

ous poly(e

hibited no

y available p

xcellent lon

to non-fluo

the imidize

yimide film

act

(°)

D

a

5

2

2

3

Q-6FDA-i 3±3o).

epared by th

-(4'-amino b

inated and

ether imide

t only l

poly(ether i

ng term th

orinated dia

ed aligned n

ms

Dielectric constant at 1 MHz

2.901

2.861

2.957

2.787

polyimide

he reaction

benzoxy)be

non-fluorin

e)s contai

low diele

imide) ULT

hermo-oxida

anhydride-b

nano

mat

of a

enzyl]

nated

ning

ctric

TEM

ative

ased

54  

polyimides. Such materials could be of high use as insulating material in interlayer

dielectrics besides their use in filter and composite industry.

The details of this work has already been published in Polymers for Advanced

Technologies and attached in Appendix 8.5.

55  

4.5.2 Declaration of my contribution

The synthesis of monomer 4,4-Bis[3'-trifluoromethyl-4'-(4'-amino benzoxy)benzyl]

biphenyl and dielectric constant test was done in Prof. Dr. Susanta Banerjee’s group

by Debaditya Bera. Prof. Dr. Seema Agarwal supported the whole project, proposed

many useful suggestions and helped in writing the introduction part of the manuscript.

The preparation of electrospun nanofibers and characterization, writing the draft of

manuscript were done by me.

56  

5. Outlook Stimuli responsive materials and biodegradable polymers have been received

increasing interest in the scientific as well as the technical community as they have

big potential applications in biosensors, tissue engineering, drug delivery and many

more fields. By proper choice of processing technique such as electrospinning, it is

possible to produce nanostructured materials from these functional polymers. Due to

novel characteristics in the nanosized dimension, the functional materials possess

some new unique properties which surely will pave the way for more promising

applications in biomedical fields. Thus the study in this work emphasized on the

preparation and characterization of some functional nanomaterials by electrospinning

and heterophase polymerization. The fundamental work presented here provides some

concepts for making novel functional nanostructured materials and surely a large

number of possible research directions could be developed based on this work.

For stimuli responsive superabsorbent nanomats, the water absorption / desorption

was controlled by photo irradiation due to photo responsivity of the embedded

particles; the influence of particle size on photo-responsive rate should be further

investigated. For a real biosensor application, the desorption rate of these

nanocomposites should be optimized and other hydrophilic biodegradable elastomers

could be a good choice as substrate. For the antibacterial segmented block

copolyurethane nanoparticles with UCST behavior, the full understanding of UCST

behavior influenced by the degree of quaternization and the pH should be further

investigated. For smart drug encapsulation and release, a proper hydrophilic drug

needs to be chosen to investigate the drug release profile in vitro. As a choice, PCL

segments could be introduced into the copolymer to impart the biodegradability of

this particle system for further biomedical applications.

In the second main part of this dissertation, novel odd-odd polyester and PHA-b-PEO

copolyesters were synthesized and enzymatic degradation results were shown. The

polyester-5,7 showed a similar crystal structure and degradation mechanism with

57  

commercial available PCL; the crystallization process and crystallization kinetics

need to be investigated to determine the self-nucleation temperature. For a real

biomedical application, the cytotoxicity test and in vivo degradation mechanism

deserve further study. The PHA-b-PEO nanofiber by “green electrospinning”

technique offers novel perspectives for application in medicine, pharmacy and

agriculture, but still many work deserves further research including the

biodegradation behavior of such “nanoparticles formed nanofiber”, the influence of

molecular weight, PHA/PEO block ratio and block length should also be investigated

to get a structure-property relationship for further applications.

The last part of this thesis showed successful preparation of low dielectric constant

nanomats by proper combination of fluorinated polyimides with electrospinning

processing technique. The mechanism needs to be proposed to explain the low

dielectric constant, other fluorinated diamine monomers can be chosen for further

investigation.

58  

6. Acknowledgements First and foremost, I would like to thank my direct supervisor prof. Dr. Seema

Agarwal, for giving me this interesting research topic. I am deeply indebted to her

guidance in my work. Without her continued support, I would not have been possible

to achieve my goal during scheduled time. She showed me how to initiate and

approach a research problem together with teaching me how to organize the results

and the art of writing scientific paper.

I would also like to thank Prof. Dr. Andreas Greiner for his kindness and all the useful

suggestions, his open mind and concern inspire me a lot during my entire PhD

episode.

I want to give my gratitude to Prof. Dr. Haoqing Hou in the department of chemistry,

Jiangxi Normal University, for his recommendation and continued concerning with

my work.

Thanks also goes to the secretary of our group, Mrs. Edith Schmidt, for her numerous

and patient help with the official work.

I want to thank all my former and present working group members for their kind help

during my research and for making the group a wonderful work place. Firstly, I am

very thankful to my lab mates: Claudia Mattheis, Christoph Luy and Qiao Jin for their

friendly nature and kind help, the lab become an enjoyable work place because of all

your participation. I gratefully acknowledge Claudia Mettheis, Ilka Elisabeth Paulus

for their valuable assistance in reviewing my thesis so patiently. I owe my special

thanks to Dr. Roland Dersch for his constructive advice and kind help. My thanks

then go to Anna Bier and Martina Gerlach for their kind help in ordering chemicals

for my work. Next, thanks go to Christoph Luy and Andreas Hedderich for helping

me tackle all the computer related problems. Thanks then to Ilka Elisabeth Paulus, Yi

Zhang, Christian Heel and Stefan Boken for showing me the MDSC technique and

lots of GPC measurements. Thanks also go to Elisabeth Giebel for showing me the

59  

mechanical tests, Jan Seuring for the DLS and polarized optical microscopy

measurement. Thanks for Uwe Justus for his useful suggestions during my work.

I am grateful to Prof. Dr. Susanta Banerjee and his student Debaditya Bera in the

materials science center, Indian Institue of Technology, Kharagpur, for their

cooperation to finish the project “Low dielectric constant polyimide nanomats by

electrospinning”, it was a wonderful experience cooperated with Prof. Dr. Susanta

Banerjee.

Special thanks also go to the former group member Jan Martin Nölle, for his synthesis

work of polyester-5, 7 for my enzymatic degradation investigation. I am also grateful

to Prof. Dr. Martin Koch and his students Steffen Wietzke, Marco Reuter and Sangam

Chatterjee for measuring the glass transition temperature by terahertz time-domain

spectroscopy (THz TDS).

I am also grateful to Kathrin Bubel and former co-worker Jinyuan Sun for their

cooperation to successfully finish the project “Nanofibers by green electrospinning of

aqueous dispersions of biodegradable blockcopolyester for application in medicine,

pharmacy and agriculture”.

Thanks my master practical student Yu Su for her passionate participation in part of

my projects .

Thank Michael Hellwig and Dr. Andreas Schaper for helping me the electron

microscope measurements.

I cannot forget to all my friends in Germany and China who always have been a

source of inspiration. The valuable memories with their company, I will cherish

throughout my life.

Finally, I want to thank my family: my parents for giving me life in the first place, for

their unconditional support and affection never returned. I would also like to thank my

elder brother and sister for their support my studies. My special thanks go to Yin Jian,

60  

my girlfriend, for her love and support for my studies, without her encourage and

understanding, I would have never concentrated on my work and finished my work in

due time.

61  

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quantum dots: a low-cost photovoltaic material. Energy & Environmental

Science, 2010. 3(12): p. 2010-2014.

50. Mukherjee, K., et al., Electron transport in electrospun TiO(2) nanofiber

dye-sensitized solar cells. Applied Physics Letters, 2009. 95(1).

51. Yang, S.Y., et al., Electrospun TiO2 nanorods assembly sensitized by CdS

quantum dots: a low-cost photovoltaic material. Energy & Environmental

Science, 2010. 3(12): p. 2010-2014.

52. Chen, S.L., et al., Electrospun and solution blown three-dimensional carbon

fiber nonwovens for application as electrodes in microbial fuel cells. Energy &

Environmental Science, 2011. 4(4): p. 1417-1421.

67  

8. Appendix

 

68  

8.1 Publication “Stimuli-Responsive Elastic Polyurethane-Based Superabsorber

Nanomat Composites”

Fei Chen, Andreas Greiner, Seema Agarwal*, Stimuli-Responsive Elastic Polyurethane-Based Superabsorber Nanomat Composites, Macromol. Mater. Eng. 2011, 296, 517–523

69  

70  

71  

72  

73  

74  

75  

76  

8.2 Manuscript “Multifunctional Polyurethane Aqueous Dispersions showing

Thermo Responsivity with UCST and Antibacterial Properties”

Fei Chen, Judith Hehl, Yu Su, Claudia Mattheis, Seema Agarwal*, Multifunctional Polyurethane Aqueous Dispersions showing Thermo Responsivity with UCST and Antibacterial Properties, Journal of Colloid and Interface Science. 2011, Submitted.

77  

Multifunctional Polyurethane aqueous Dispersions showing Thermo

Responsivity with UCST and Antibacterial Properties

Fei Chen, Judith Hehl, Yu Su, Claudia Mattheis, Seema Agarwal*

Philipps-Universität Marburg, Department of Chemistry and Scientific Center for

Materials Science

Hans-Meerwein Strasse, D-35032, Marburg, Germany

*seema@chemie.uni-marburg.de

Abstract: Cationic segmented polyurethane dispersions with UCST behavior and

antibacterial properties are highlighted in this article. The base polyurethanes were

prepared by polyaddition reaction followed by quaternisation with methyl iodide.

Aqueous dispersions were made by simple heating at high temperature and slow

cooling. No additional emulsifiers / stabilizers or special procedures were required to

obtain aqueous stable dispersions of solid contents of up to 10% wt/wt. The

dispersions depending upon their composition showed high efficiency and fast time of

action as antibacterial material. Particle size and UCST strongly depended on the

solid content of the dispersions and the content of PEG segments in the copolymers.

Such novel UCST, antibacterial dispersions have a big potential to be used for drug

encapsulation and controlled release for various therapeutic applications besides

general use as water stable coating at room temperature.

Keywords: polyurethane, antibacterial, dispersion, UCST

78  

Introduction

Aqueous polyurethane dispersions are known since the late 1960s and have attracted

both academia and the industry. The conventional PUs is water insoluble and requires

strong shear forces and external emulsifiers for making water dispersion[1]. In the last

few years modified PUs were made with an aim to provide self-dispersibility under

mild conditions. This could be done by either introducing hydrophilic segments or

ionic moieties in PU structure. For example, the introduction of a small amount of

ionic charges into polyurethanes combine the properties of the parent polymers with

those derived from the existence of ionic structures in the backbone, offering a big

potential to form aqueous dispersions for different applications such as coating and

hygiene products[2-4]. Different types of PUs depending on the nature of the charge

(positive, negative or both) placed within the polymeric backbone, pendant or at the

end of PU chains are already known [5-9]. The particle size of the dispersion is

dependent to a large extent on the concentration of ionic groups[10]. Cationic PU

dispersions show very high adhesion to various ionic substrates, especially anionic

substrates such as leather and glass[11]. Also, besides the important application of

cationic dispersions in synthetic leather industry, it has been reported that various

polycations possess antibacterial properties in solution, presumably by interacting

with and disrupting bacterial cell membranes[12]; due to this very general mode of

action, these compounds can be used to kill a wide range of bacteria and the activity

can potentially be recovered by removal of dead cells from the polymer surface[13].

The synthesis of PU dispersions is mainly carried out by stirring a solution of PU in

organic solvent like acetone and methyl ethyl ketone with water i.e. solvent

displacement method [14, 15]. Other highly used methods are: prepolymer mixing

process and melt dispersion process. Landfester et al. showed the use of miniemulsion

polymerizatiuon for making PU dispersions with diameters of about 200 nm[16].

The common drawback of these methods is that normally emulsifier is needed to

stabilize the dispersion, or in the raw material stage of cationic polyurethane synthesis,

79  

volatile organic solvents are necessary in the preparation and dissolution of

prepolymers. Organic solvents are harmful for many reasons such as toxicity and

flammability, and thereby cause severe concern in production with respect to

environmental and safety issues. On the other hand, the hydrophilic modification of

PUs for making it suitable as self-emulsifier leads to poor water resistance. This can

be overcome by hydrophobic modifications or cross-linking of PU chains, but both of

these strategies are time consuming and complicated synthetic route need to be

adopted[17, 18].

Compared to hydrophobic modification or cross-linking of PU chains, employment of

UCST (upper critical solution temperature) behavior would be another choice to

improve the water resistance. The dissolved polymer chains undergo a change in

conformation from coil to globule with temperature and thus undergo phase

separation. Moreover such smart dispersions are always desirable for controlled

release applications. Polymers exhibiting a sharp and reproducible change in physical

properties upon a small change in the environment, e.g. temperature, ionic strength,

pH, mechanical stimuli are classified as stimuli-responsive or also “smart”

polymers[19]. A class of very prominent stimuli-responsive polymers are

thermoresponsive polymers including the well-investigated and well-known

poly(N,N-isopropylacrylamide) (PNiPAAm)[20]. Above 33 °C an aqueous solution of

PNiPAAm shows a so called LCST behavior (lower critical solution temperature), i.e.

phase separation[21]. UCST behavior (upper critical solution temperature) is much

less well-known for polymeric materials than LCST. Only few examples can be found

in literature [22-28]. Inverse emulsion polymerization of acrylamide and acrylic acid

was used for making UCST microgels by Mijangos et al [29].

Here we report for the first time (to the best of our knowledge) a stable dispersion

showing UCST based on the cationic polyurethanes. The proper choice of the

alkylating group for quaternization and the use of poly(ethylene oxide) segments led

to the self emulsification of cationic PUs. This facilitated the formation of a stable

80  

dispersion without the use of external stabiliying agent. The facile preparation of

cationic PU dispersions, the influence of copolymer composition on the dispersion

formation and stability, antibacterial activity and UCST is reported here. The

antibacterial nature of smart UCST dispersion could be promising for use in water

stable coatings etc.

Experimental

Materials

2,4-toluenediisocyanate (TDI, Sigma-Aldrich, 95%) and diethanol-N-methylamine

(DEMA, Acros, 99%+) were distilled under vacuum and stored under argon.

1,4-diazabicyclo[2.2.2]octane (DABCO, Sigma-Aldrich, 98%) was recrystallized

from cyclohexane and stored under argon. Polyethylene glycol (PEG 2000) (Merck)

was purified by dissolving in dichloromethane, precipitating in diethyl ether and

drying in a vacuum oven. Tetrahydrofuran (THF, BASF, techn.) was pre-dried by

distillation over potassium hydroxide, then dried over phosphorous pentoxide,

distilled and stored under argon. Cyclohexane (BASF, techn.), hexane (BASF, techn.),

methanol (BASF, techn.) and N,N-dimethyl formamide (DMF, BASF, techn.) were

purified by distillation. Phosphorus pentoxide (Riedel-de Haёn, 98.5 %), potassium

hydroxide (Sigma-Aldrich, techn.), methyl iodide (Sigma-Aldrich, 99%), were used

as received.

Characterization

Methods

The molecular weights and molecular weight distributions of the polymers (c = 1 g/L)

were determined by size exclusion chromatography (SEC) using a PSS GRAM linear

50 mm · 8 mm column (10 µm) and two PSS GRAM linear 600 mm · 8 mm columns

(100 Å and 3000 Å, respectively) at 25 °C and a refractive index detector (Knauer),

using DMF or LiBr/DMF (5 g/L) as eluent at a flow rate of 1 mL/min. Polystyrene

81  

standards were used for calibration with toluene as internal standard. The WinGPC

Unity (5403) Software (PSS) was used for data recording and interpretation.

1H and 13C NMR spectra were recorded on Bruker Avance 300 A and Bruker DRX

500 spectrometers respectively, using deuterated dimethylsulfoxide (d6-DMSO) as

solvent. Topspin 3.0.b.7 (Bruker) was used for data interpretation. For calibration the

residual signal of the deuterated DMSO was set to δ = 2.50 ppm (1H NMR) and

δ = 39.5 ppm (13C NMR).

FTIR spectra were recorded on a Digilab Excalibur Series system using a Pike

Miracle attenuated total reflection (ATR) unit. The samples were measured in

powdered state without further preparation necessary. The Win-IR Pro 3.3 software

(Digilab) was used for data recording and interpretation.

The particle size and morphology were characterized by scanning electron

microscopy using a field emission scanning electron microscope JSM-7500F (JEOL)

at acceleration voltages of 2-5 kV equipped with an ALTO-2500 LN2-Cryo-transfer

system (Gatan) and a YAG-BSE detector (Autrata) aso by Delsa™ Nano C particle

analyzer.

UCST behavior of the dispersion was characterized by turbidity measurements using

a TP1-D turbidity photometer from TEPPER-Analytic. The measurements were

performed at a wavelength of 670 nm and a heating and cooling rate of 1°C/min in a

temperature range from 90°C to 60°C. The cyclic temperature program was repeated

2 times. Measurements were carried out at constant stirring in a glass cuvette of 10

mm path length.

General synthesis of base polymer (PU-0)

A dry three-necked round bottom flask equipped with a dropping funnel and argon

inlet was charged with TDI (71.17 g, 408.7 mmol, 1eq) and THF (500 ml). The

resulting solution was cooled down to -50 °C by a dry-ice/isopropanol mixture. A

solution of DEMA (48.7 g, 408.6 mmol, 0.99 eq) in THF (300 ml) was transferred

82  

into the dropping funnel and added dropwise to the TDI solution at -50 °C. The

DABCO catalyst (2.290 g, 20.42 mmol, 0.05 eq) was added to the reaction mixture

which was subsequently warmed to room temperature and then 50 °C. After 3.5 h the

polyaddition was stopped by cooling to room temperature and addition of methanol

(50 ml). The polymer P(TDI-DEMA) (PU-0) was precipitated from hexane (6 l).

Yield: 117 g (98%), T5% = 220 °C. Tg = 77 °C, Mp = 30500 Da, 1H-NMR (500 MHz,

d6-DMSO), δ/ppm: 9.57 (s, 1H, N8aH), 8.82 (s, 1H, N8bH), 7.51 (s, 1H, C3H), 7.17 (d,

1H, C5H, J = 8 Hz), 7.06 (d, 1H, C6H, J = 8.5 Hz), 4.14 (s, 4H, C10a/bH2), 2.68 (s, 4H,

C11a/bH2), 2.30 (s, 3H, C12H3), 2.11 (s, 3H, C7H3). 13C-NMR (125 MHz, d6-DMSO),

δ/ppm: 154.1 (C9a/b=O), 153.7 (C9a/b=O), 137.3 (C4H), 136.7 (C2H), 129.8 (C6H),

126.4 (C1H), 115.5 (C3H und C5H), 62.3 (C10a/bH2), 61.4 (C10a/bH2), 56.0

(C11a/bH2), 42.3 (C12H3), 17.2 (C7H3). FTIR (ATR), ν/cm-1: 3298w (ν-NHCOO-),

2963w, 2804w, 1701s (ν(C=O), Amide-I), 1601m, 1531s (Amide-II), 1450m, 1223s,

1049s, 880w, 768m (Amide-V), 567w.

General synthesis of quaternized polymer (QPU-0)

P(TDI-DEMA) (15.03 g, 0.051 mol, 1.00 eq) was dissolved in DMF (300 ml) and the

methyl iodide (6.5 ml, 0.104 mol, 2.03 eq) added. The reaction mixture was stirred at

70 °C for 48 h. The fully quaternized polymer QPU-0 was precipitated from THF

(4.5 l). For further purification the crude, yellow product was extracted with THF

(1.5 l) for 48 h. The product was obtained as white powder. Yield: 22g (99%) ,

T5% = 217 °C, Tg = 165 °C, Mp = 10446 Da, 1H-NMR (300 MHz, d6-DMSO), δ/ppm:

9.72 (s, 1H, N8aH), 9.01 (s, 1H, N8bH), 7.57 (s, 1H, C3H), 7.21 (d, 1H, J = 8.5 Hz,

C5H), 7.13 (d, 1H, J = 8.9 Hz, C6H), 4.55 (s, 4H, C10a/bH2), 3.81 (s, 4H, C11a/bH2),

3.24 (s, 6H, C12H3 und C13H3), 2.15 (s, 3H, C7H3). 13C-NMR (76 MHz, d6-DMSO),

δ/ppm: 153.3 (C9a/b=O), 152.5 (C9a/b=O), 136.7 (C4H), 136.0 (C2H), 130.5 (C6H),

126.2 (C1H), 115.1 (C3H und C5H), 63.0 (C10a/bH2), 57.8 (C11a/bH2), 51.5 (C12H3,

C13H3), 17.1 (C7H3). FTIR (ATR), ν/cm-1: 3300w (ν-NHCOO-), 2884m, 1705s

83  

(ν(C=O), Amide-I), 1602m, 1529s (Amide-II), 1450m, 1223s, 1056s, 878w, 762m

(Amide-V).

General synthesis of co-polyurethane (PU-X)

The typical procedure for synthesis of co-polyurethane (PU-3) with 3 mol% of

PEG2000 in the copolymer was described here. A dry three-necked round bottom

flask equipped with a dropping funnel and argon inlet was charged with TDI (2.25

mL, 15.76 mmol, 1.00 eq) and THF (32 mL). The resulting solution was cooled down

to -50 °C by a dry-ice/isopropanol mixture. A solution of DEMA (1.82 g, 15.27 mmol,

0.97 eq) in THF (300 mL) was transferred into the dropping funnel and added

dropwise to the TDI solution at -50 °C. The DABCO catalyst (0.092 g, 0.79 mmol,

0.05 eq) and PEG 2000 (0.95g, 0.47 mmol, 0.03 eq) was added to the reaction mixture

which was subsequently warmed to room temperature and then 50 °C. After 3.5 h the

polyaddition was stopped by cooling to room temperature and addition of methanol

(50 mL). The polymer PU-3 was precipitated from hexane, the product was heated at

45 °C under vacuum for two days.

General synthesis of quaternized polymer (QPU-X)

PU-3 (0.60g, mol, 1.00 eq) was dissolved in DMF (10 mL) and the methyl iodide

(0.3 mL, mol, 2.03 eq) was added dropwise to the mixture. The reaction mixture was

stirred at 70 °C for 48 h. The fully quaternized polymer QPU-3 was precipitated from

THF (500 ml). For further purification, the crude yellow product was extracted with

THF (1.0 l) for 48 h. The product was obtained as white powder. Yield: 22g (99%).

Base homo-polyurethane and quaternized polyurethane were denoted as PU-0 and

QPU-0, co-polyurethane and quaternized co-polyurethane were named as PU-X and

QPU-X, with X referring to the molar ratio of PEG2000 in the copolymers.

Microbiological testing

By inoculation of a TSB (tryptic soy broth, Sigma Aldrich, aqueous solution c =

30 g/l) solution with a single colony of E. coli (DSM No. 1077, K12 strain 343/113),

84  

incubation with shaking at 37 °C until the optical density at 578 nm had increased by

0.125 and dilution 1:99 with sterile liquid broth the E. coli inoculum was prepared.

The exact cell density was determined by spreading serial tenfold dilution on nutrient

agar plates, incubation for 24 h at 37 °C and colony counting.

The antibacterial activity of biocides is characterized by the minimum inhibition

concentration (MIC), that is defined as the minimum amount of a substance which is

required to inhibit bacteria growth, as well as the minimum bactericidal concentration

(MBC), that gives the minimal concentration of a substance that kills 99.9% of the

bacteria (4 log stages). For the determination of the MIC, a geometric dilution series

of 1 % wt/wt dispersions in TSB was inoculated with an E. coli inoculum in a sterile

24 well plate and incubated for 24 h at 37 °C. The wells were visually evaluated for

bacteria growth; the lowest dilution stage which did not show bacteria growth, i. e.

did not become turbid, was taken as MIC; each test was done in duplicate. To

determine the MBC, 100 µl of the dispersions which remained clear were spread on

nutrient agar plates and incubated at 37 °C for 24 h. The lowest dilution stage which

did not lead to colony formation was defined as MBC. The time-depending

antibacterial activity was determined by spreading 100 µl specimens of the MIC

dispersions after given time intervals on nutrient agar plates. After incubation at 37 °C

for 24 h the colonies were counted; the reduction was calculated respective to the

inoculum.

85  

Results and Discussion

Quaternized polyuretnanes based on 2,4-toluenediisocyanate (TDI) and

diethanol-N-methylamine (DEMA) were used for making functional dispersions. The

polyurethanes were made by polyaddition reaction and were quaternized using methyl

iodide (Scheme 1) by a simple SN2 reaction. By using the low molecular weight

DEMA which included the desired quaternizable amine unit as diol, a maximum

amount of ionic groups in the final polymer could be facilitated. The resulting PUs

were characterized using NMR and IR spectroscopy. To ensure full quaternisation

excess alkylation agent and long reaction times were employed.

OCN NCO HON

OH

TDI DEMA

THFDABCO

OO

HN

HN

O

ON

RXDMF

n

OO

HN

HN

O

ON

R

X

n

Scheme 1: Synthesis of the base polymer PU-0 and quaternized QPU-0 from 2,4-toluene diisocyanate (TDI) and diethanol-N-methylamine (DEMA).

The quaternized PU (QPU-0) showed upper-critical solution-temperature (UCST)

behavior. The polymer was not soluble in water at room temperature but showed

solubility in water on heating depending upon the polymer concentration at a very

high temperature (98 °C). A polymer concentration of less than or equal to 5 wt% led

to a clear solution of PUs in water at about 98 °C. The simple cooling of this solution

to room temperature (20 °C) without stirring yielded opaque dispersions. The

86  

dispersion was characterized by dynamic light scattering (DLS) and showed average

particle size of about 130 nm. UCST was determined by turbidity measurements and

found to be about 80oC. Above this temperature the polymer made a clear solution

and below it made a stable dispersion. The process was completely reversible. This

process of using UCST behavior to obtain secondary dispersions is much easier

compared to classic processes, e.g. the solvent displacement method[30]. In this

process the material is dissolved in an organic water-miscible solvent of low boiling

point and water added to the solution. The organic solvent is then subsequently

evaporated from the dispersion[31]. Other means to produce secondary dispersions

often employ special mixing techniques, e.g. sonification, high speed stirring or

homogenization[32]. Typical procedures also often use additives, e.g. surfactants, to

facilitate secondary dispersions. The dispersions made in this work using UCST

behavior are surfactant free which is desirable for many different biomedical and

agricultural applications. To further increase the solid content of the dispersion, we

designed segmented PUs with PEG (polyethylene glycol) segments with increased

hydrophilicity (Scheme 2).

OCN NCO HON

OHOOHH n

OO

HN

HN

O

ONO

O

HN

HN

O

On p q

PEG2000 TDI DEMATHFDABCO

RXDMF

OO

HN

HN

O

ONO

O

HN

HN

O

On p qR

X

Scheme 2: Synthesis route of the base copolymer PU-X and quaternized

co-polyurethanes QPU-X

87  

The synthesis of the segmented block co-polyurethanes PU-X was accomplished by

the same method as described above for PU-0 in Scheme 2. Compared with the

homo-polyurethane PU-0 synthesis, additionally a PEG polyol with a number average

molecular weight of 2000 g/mol was employed. By varying the ratio between DEMA

and PEG2000, a series of segmented block co-polyurethanes were synthesized with

PEG ranging from 0-10 mol%. The base copolymers PU-X were characterized by

nuclear magnetic resonance (NMR), infrared spectroscopy (IR), size exclusion

chromatography (SEC) and differential scanning calorimetry (DSC). Figure 1 shows

the 1H-NMR spectrum of PU-X with 10 mol % PEG in the copolymer, as an example.

A peak at 3.51ppm, which is assigned to CH2 groups from PEG was observed which

showed the successful incorporation of PEG in PUs. The copolymer composition

was determined using the peak integrations of CH2 protons of PEG (3.51 ppm) and

-NCH3 protons (2.3 ppm) of DEMA. In NMR, no signs of urea linkages, only

urethane linkages were observed in the copolymers.

To introduce quaternary ammonium moieties into the copolymer, the tertiary amine

groups were quaternized with methyl iodide according to Scheme 2. Again, excess

methyl iodide and long reaction times (48 h) were employed to insure full

quaternization for the whole series of copolyurethanes. Figure 2 shows the 1H-NMR

spectrum of the copolymer QPU-X with 10 mol % of PEG. A significant shift of the

protons 10a/b, 11a/b close to the quaternized nitrogen atom could be observed. No

peaks were present at the original positions, thereby showing compelte quaternization.

The weight average molecular weight of the copolyurethanes as determined by gel

permeation chromatography in DMF with LiBr was about 22,600 but with broad

polydispersity.

88  

Figure 1: 1H-NMR of PU-10 in d6-DMSO.

Figure 2: 1H-NMR of QPU-10 in d6-DMSO.

89  

Figure 3 FT-IR spectra of (A) PEG 2000, (B) PU-10 and (C) PU-0.

Representative FT-IR spectra of PEG 2000, co-polyurethane and homo-polyurethane

are shown in Figure 3. The spectra showed characteristic bands of urethane >NH at

3298 cm–1 and >C=O stretching of urethane linkage at 1701 cm-1. The symmetric and

asymmetric stretching of CH2 and C-O-C stretching vibration at 1115 cm-1 of PEG

was observed between 3000 and 2750 cm-1. The aromatic >C=C< band of TDI was

observed around 1600 cm-1. The band at 1540 cm-1 is due to the C-N stretching and

NH deformation. When the block copolymers were converted to their cationomers,

the tertiary nitrogen atoms were converted to quaternary ammonium groups, which

gave rise to peaks around 980–930 cm-1, characteristic for aliphatic quaternary

ammonium salts, see Figure 4.

90  

Figure 4. FT-IR spectra of PU-5 (A) before and (B) after quaternization with CH3I.

It was interesting to investigate the influence of PEG on the solubility and stability of

the dispersions compared with the quaternized homo-polyurethanes. Results indicated

that the solubility of the copolymers increased with increase in the content of PEG in

the copolymers (Table 1). When the content of PEG in the copolymers increased to

10 mol% (QPU-10), the copolymers could not form stable dispersions anymore. This

is due to the high hydrophilicity of the copolymer and therefore remained in solution.

Therefore, an optimum amount of PEG (about 5 wt %) was required in

co-polyurethanes to make stable dispersions with high solid contents. The decrease in

amount of PEG led to decreased solid content and an increase in its amount in

co-polyurethanes led to disappearance of the UCST behavior.

91  

Table 1: Dispersion experiments with the cationic polyurethanes.

+: soluble -: insoluble *: dispersion & gel like precipitate

QPU-X C(PU)

/% wt/wt

Solubility Dispersion

QPU-0 ≤5

10

+

-

*

QPU-1 >1≤8 + *

QPU-3 >1≤10 + *

QPU-5 >1≤12 + *a

QPU-10 >1≤20 + solution

a: quaternized co-polyurethane(QPU-5) could form dispersion only above 5wt%.

The dispersions were characterized by dynamic light scattering (DLS) and turbidity

photometry. Particle sizes were determined as a function of the PEG content and

concentration of the aqueous dispersions of quaternized co-polyurethanes by DLS

(Table 2). An increase in particle size was observed with increasing dispersion

concentration, for example, the particle size increased from 70 nm at 1wt % to around

1000 nm at 8 wt % for QPU-1 (Figure 5). This could be due to the repulsion between

the ionic moieties.

92  

100 1000 100000

5

10

15

20

Diff

eren

tial I

nten

sity

(%)

Diameter(nm)

1% 2% 3% 4% 5% 6% 8%

Figure 5. Effect of dispersion concentration for QPU-1 on Particle size.

The particles size measurement by DLS was also done for the same concentration of

co-polyurethane with different PEG ratios. Figure 6 showed that there is no big

difference of particle size among all the samples, the only change was that by

increasing the PEG content in the copolymer, the particle size distribution decreased

but the particle size kept constant around 80 nm.

10 100 10000

5

10

15

20

25

Diff

eren

tial I

nten

sity

(%)

Diameter(nm)

QPU-5 QPU-3 QPU-1 QPU-0

Figure 6. Effect of PEG (QPU-0, QPU-1, QPU-3 and QPU-5) content on particle

size.

93  

Table 2 Particles size of quaternized PU dispersions as measured by dynamic light scattering

QPU Diameter

/nm

QPU Diameter

/nm

1% QPU-0 128±97 1% QPU-1 71±26

1% QPU-1 71±26 2% QPU-1 167±53

1% QPU-3 100±35 3% QPU-1 369±164

1% QPU-5 136±58 4% QPU-1 358±94

5% QPU-1 563±28

6% QPU-1 1054±122

8% QPU-1 863±93

The particle shape was characterized by scanning electron microscopy. Figure 7

shows the morphology of 1 wt% of quaternized QPU-0 and QPU-1. Without dilution,

the particles form a film; the diluted samples possessed almost spherical shape with

some particles aggregation.

Figure 7: SEM images of a dried (A) diluted and (B) undiluted dispersion of

QPU-0(1% wt/wt); (C) diluted QPU-1 dispersion.

The UCST behavior of the QPU dispersions was investigated using turbidity

photometric measurements (Table 3). Figure 8 shows the obtained turbidity curves

for QPU-1 dispersions with different concentrations. At low concentration (1wt %)

B A  C

94  

the UCST was not sharp. Increase in the concentration led to significant and

systematic increase in UCST (from 48 °C at 1 wt% to 81°C for 6 wt %) with sharp

phase change. All dispersions showed good reproducibility of their respective UCST

behavior over at 5 heating/cooling cycles (data not shown here). The UCST behavior

of quaternized polyurethane dispersions with the same concentration but with

different contents of PEG in the composition was also studied (Figure 9). There was

no significant influence of PEG in changing the UCST but led to stable dispersions

with high solid contents.

0 20 40 60 80

0

20

40

60

80

100

120

140

Rel

ativ

e In

tens

ity

Temperature(°C)

1% 2% 3% 4% 5% 6%

Figure 8: Turbidity measurements for the investigation of the UCST behavior of the

iodide derivatives of the cationic QPU-1 with different concentrations.

95  

70 72 74 76 78 80 82 84 86 88 90

0

20

40

60

80

100

120

Tran

smitt

ance

(%)

Temperature(°C)

QPU-0 QPU-1 QPU-3 QPU-5

Figure 9. Turbidity measurements of 5 wt% quaternized PU with different PEG

contents to investigate the influence of PEG on the UCST behavior of copolymers

(first cooling cycle).

Table 3 Upper Critical Solution Temperature (UCST) of PU dispersions as measured

by turbidity photometer.

QPU UCST

/ °C

QPU UCST

/ °C

5% QPU-0 76 1% QPU-1 48

5% QPU-1 83 2% QPU-1 63

5% QPU-3 84 3% QPU-1 77

5% QPU-5 82 4% QPU-1 78

5% QPU-1 83

6% QPU-1 81

The antibacterial properties of the dispersions were investigated by several standard

methods. All dispersions proved to be active against E. coli; the determined MIC and

MBC values do not differ among the tested dispersions (see Table 4) and are with

96  

concentrations of 78 µg/ml as MIC and 156 µg/ml as MBC in a good range. When the

focus comes to the speed of antibacterial action, the quaternized homo-polyurethane

(QPU-0) dispersion showed to be the best: already after 10 minutes of contact, the

solution with a concentration of 2.5 mg/ml and all higher concentrated samples killed >

99.9% of the bacteria, as shown in Figure 10A. After a longer contact time of 120

minutes also the other dispersions showed a total reduction of bacteria growth at

concentrations of ≥ 625 µg/ml or even ≥ 313 µg/ml, when it comes to the QPU-0

dispersion, as depicted in Figure 10B.

Table 4. Minimum inhibition concentration (MIC) and minimum bactericidal

concentration (MBC) of the dispersions towards a 105 cfu/ml suspension of E. coli.

Sample MIC / µg ml-1 MBC / µg ml-1

QPU-0 78 156

QPU-1 78 156

QPU-3 78 156

QPU-5 78 156

97  

Figure 10. Reduction of bacteria growth for different concentration of the dispersions

after (A) 10 minutes and (B) 120 minutes contact to E. coli (105 cfu/ml).

Conclusions

A simple and straightforward method of making UCST PU dispersions with high

solid contents could be shown here. Cationic segmented polyurethanes having PEG

segments with UCST behavior and antibacterial properties were successfully

synthesized by polyaddition reaction followed by quaternization. The introduction of

PEG into the system facilitated the formation of water stable dispersion with high

solid contents up to 10 wt% without any additives. The dispersed particles are well

below 1μm in size and comprise a low size distribution. Particle size and UCST could

be easily controlled by the solid concentration and PEG content. More importantly,

the introduction of PEG into the copolymer did not decrease the antibacterial activity.

Low MIC and MBC against the gram-negative Escherichia coli suggest high

suitability of the dispersions for functional coatings and antibacterial textile

applications.

Acknowledgements: Financial support from department of Chemistry

Philllips-University Marburg and BMBF, Germany is highly acknowledged.

 

98  

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100  

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101  

8.3 Publication “A Fast Degrading Odd-Odd Aliphatic Polyester-5,7 made by

Condensation Polymerization for Biomedical Applications”

Fei Chen, Jan Martin Nölle, Steffen Wietzke, Marco Reuter, Sangam Chatterjee, Martin Koch, Seema Agarwal*, A Fast Degrading Odd-Odd Aliphatic Polyester-5,7 made by Condensation Polymerization for Biomedical Applications, Journal of Biomaterials Science: Polymer Edition. 2011, in press.

102  

A Fast Degrading Odd-Odd Aliphatic Polyester-5,7 made by

Condensation Polymerization for Biomedical Applications

Fei Chena, Jan Martin Nöllea, Steffen Wietzkeb, Marco Reuterb, Sangam Chatterjeeb,

Martin Kochb, Seema Agarwala*

aPhilipps-Universität Marburg, Fachbereich Chemie, Hans-Meerwein Strasse, D-35032, Marburg, Germany bPhilipps-Universität Marburg, Fachbereich Physics, Hans-Meerwein Strasse, D-35032 Marburg, Germany agarwal@staff.uni-marburg.de

Abstract

A fast enzymatic degradable aliphatic biodegradable all-odd-polyester-5,7 based on

1,7-heptanedioicacid (pimelic acid) and 1,5-pentanediol was synthesized by

polycondensation reaction. The structural characterization of the polyester was done

using 1D and 2D NMR spectroscopic techniques. The properties of the resulting

polyester like crystallization behavior, enzymatic degradation, thermal stability etc.

were investigated by using differential scanning calorimetry (DSC), wide angle X-ray

diffraction (WAXD), scanning electron microscopy (SEM) and gel permeation

chromatography (GPC). Terahertz time-domain spectroscopy (THz TDS) was

employed to determine the glass transition temperature, which could not be revealed

reliably by conventional thermal analysis. The properties of all-odd-polyester-5,7

were compared with the well known enzymatic degradable polyester

(polycaprolactone). The results indicated that polyester-5,7 has similar crystal

structure like PCL but much faster degradation rate. The morphology of polyester-5,7

film during enzymatic degradation showed fibrillar structure and degradation began

by surface erosion.

Key words: aliphatic polyester, condensation polymerization, degradable, enzymatic

degradability

103  

1. Introduction Large number of commercially available degradable materials as environmental

friendly materials or for medical and non-medical applications is based on aliphatic

polyesters. They are generally made by ring-opening polymerization of cyclic esters

[1-3] or polycondensation reaction between diols and diacids or diacid derivatives

[4-5]. Both methods have their own advantages and disadvantages and can be chosen

according to the application. Ring-opening polymerization of cyclic esters generally

gives high molecular weight polyesters which is not possible by polycondensation

reactions. On the other hand, polycondensation reaction provides more opportunities

of tuning the properties of polyesters by using different combinations of diols and

diacids. The use of polycondensation reactions for the synthesis of aliphatic polyesters

(poly(alkylene dicarboxylates)) is known since 1930s from the work of Carothers

[6]. In the recent times new catalysts and heat stabilizers are developed and

researched with an aim to get the high molecular weight aliphatic polyesters by

condensation polymerization. Poly(alkylene dicarboxylates like Poly(butylene

succinate) (PBSu), poly(ethylene succinate) (PESu) and poly(ethylene adipate) (PEAd)

are well characterized in terms of crystal structure and morphology [7-12]. Poly

(propylene succinate) was the first degradable polyester from 1, 3-propanediol to be

studied [13]. These aliphatic polyesters are enzymatic degradable due to their

susceptibility to hydrolysis and are degraded by different enzymes like proteases [14],

α-chymotrypsin [15], lipases and esterase [16].

From the materials point-of-view the properties and degradability rate, in general,

depends upon the type of chemical linkage, molecular weight, hydrophilicity,

crystallinity, flexibility, porosity and morphology of polyesters. The effect of these

parameters on biodegradability of polyesters made by condensation polymerization

can be seen in literature. For example, Mochizuki et al. [17] have studied the effect of

structure upon enzymatic degradation of high molar mass poly (butylenes

succinate-co-ethylene succinate). The hydrolysis rate of the copolymer was higher

than the corresponding homopolymers due to decreased crystallinity. Most of the

104  

examples in the literature made use of diols and diacids with even number of carbon

atoms. Bhaumik et al. [18] have shown the effect of odd and even carbon atoms of

diacid used on interchain packing and hence the properties like melting point of the

resulting polyesters. There was zig-zag nature of melting point variation with number

of carbon atoms (from 4 till 8) in diacid keeping ethylene glycol as the comonomer.

Similar observations were made in 1954 for different combinations of alcohols and

acids.[19] Cao et al.[20] checked degradability of poly(butylene

succinate-co-caprolactone) (caprolactone with odd number of carbon atoms) in

compost soil at 30 oC and 95% RH and showed higher degradability of copolymers as

compared to the respective homopolymers due to increased flexibility and reduced

crystallinity. Bikiaris et al. [21] have synthesized poly(alkylene succinates) from

succinic acid and aliphatic diols with 2,3 and 4 methylene groups by melt

polycondensation. These poly(alkylene succinates) are: poly(propylene succinate)

(PPSu), poly(ethylene succinate) (PESu), poly(butylene succinate) (PBSu).

Degradability studies of these polyesters with the same average molecular weight

included enzymatic hydrolysis using Rhizopus delemar lipase at pH 7.2 and 30 °C.

The degradation rates of the polymers decreased following the order

PPSu > PESu ≥ PBSu i.e. odd diol enhanced the degradation as compared to the even

ones. It was attributed to the lower crystallinity of PPSu compared to other polyesters,

rather than to differences in chemical structure. Odd carbon atoms made packing of

polymer chains difficult as compared to the even atoms and thereby led to reduced

crystallinity and increased degradability as it depends to a large extent on polymer

crystallinity.

With an aim to make and study a complete odd-polyester by polycondensation

reaction, we synthesized polyester-5,7 starting from pentanediol and

1,7-heptanedioicacid. The properties of this aliphatic all-odd-polyester including

enzymatic degradability are reported here. All-odd-polyester was found to be very fast

enzymatic degradable as compared to the very well known biodegradable polyester-

polycaprolactone (PCL). The possibility of measuring the glass transition temperature

105  

by terahertz time-domain spectroscopy (THz TDS) is also highlighted. A variety of

polymers is transparent to THz waves, which comprise the frequency range from

0.3 THz to 10 THz, tantamount to wave numbers of 10 cm−1 to 333 cm−1. Due to the

coherent detection scheme employed and the pulsed nature, THz TDS simultaneously

provides access to the dielectric properties and the thickness of the sample,

respectively, which sets it apart from techniques of the (far) infrared and the optical

frequency regime. Details about the technique and the data extraction procedure can

be found elsewhere [22].

2. Materials and Methods

2.1. Materials

1,5-pentanediol, 1,7-heptanedioicacid (Pimelic acid) were purchased from Alfa Aesar

Co., Ltd. The reagents were used as received without further purification. The

Titanium (IV) butoxide catalyst (Ti(OBu)4) was purchased from Acros and the lipase

of Pseudomonas Cepacia (50 U/mg) was purchased from Sigma-Aldrich. 0.05 M

phosphate buffer solution with pH 7.0 was prepared in the laboratory. All the other

chemicals and solvents of analytical grade were used as received without further

purification.

2.2. Synthesis of poly (pentylene heptanoate) (polyester-5,7)

The aliphatic all-odd-polyester-5,7 was synthesized by the two-stage melt

polycondensation between 1, 5-pentanediol and 1,7-heptanedioicacid (Scheme 1). The

molar ratio of 1,5-pentanediol to heptanedioicacid was 1.05:1, and the Ti(OBu)4

catalyst was 0.05 mol% of the total monomers. A representative polymerization

procedure is as follows: in a 100 ml round-bottom flask equipped with a nitrogen inlet,

a stir bar, was charged 16.0 g (100 mmol) of 1,7 heptanedioicacid (pimelic acid), 11.0

ml of 1,5-pentanediol (105 mmol) and 10.0μl (0.03 mmol) of the catalyst. After

charging into the flask, the mixture was heated to 190 °C and reacted for 2 h under

nitrogen with vigorous stirring. After this time, the calculated amount of water was

collected and then, the pressure of the mixture was gradually reduced to 0.2 mPa for

106  

30 min to avoid excessive foaming and minimize oligomer sublimation. 5mg

(0.05mmol) PPA (polyphosphoric cid) was added to the mixture and the mixture was

slowly heated to 230 °C and reacted for 6 h to obtain the polyester-5,7. After the

polycondensation reaction was complete, the polymer was cooled to room

temperature and dissolved in 50 mL of chloroform, followed by the precipitation into

800 mL of methanol. The pale-white polymer powder was collected and dried under

vacuum at 40 °C for 24 h. 1H-NMR (500 MHz, CDCl3): δ = 1.30-1.42 ppm (m, 4H,); δ = 1.62 ppm (dt, 8H, 3JH,H

= 7,5 Hz,); δ = 2.28 ppm (t, 4H, 3JH,H = 7.5 Hz); δ = 4,05 ppm (t, 4H, 3JH,H = 6.5)

2.3. Instruments 1H, 13C and 2D nuclear magnetic resonance (NMR) spectra were recorded using a

Bruker spectrometer operating at 300 MHz, the original polyester sample was

measured in CDCl3 while the degradation products were dissolved and measured in

D2O. Chemical shifts (δ) were given in ppm using tetramethylsilane(TMS) as internal

reference.

The molecular weights of the polymers were determined by GPC using a Knauer

system equipped with one column, PSS-SDV (linear XL, 5 µm, 8.0 x 300 mm), a

differential refractive index detector, CHCl3 as eluent at a flow rate of 0.5 mL/min.

All the measurements were done against the standard calibration with PMMA.

Toluene was used as internal standard. Molecular weight distributions (MWDs) of the

polymers after degradation (the water soluble portion) were determined by gel

permeation chromatography (GPC) in a setup comprising a Knauer pump equipped

with two NOVEMA column (particle size 10 µm, dimension 8.00mm × 300.00 mm,

porosity 1000 Å and 3000 Å) calibrated with poly (2-vinylpyridine)-(P2VP) standards

and a differential refractive index detector using 0.3 N formic acid as eluent with a

flow rate of 1 mL·min-1.

The surface morphology of the films were observed using JSM-7500F SEM with a

voltage of 5 kv, the dry film was directly stuck on conductive sample holder, before

107  

measurement, all the samples were coated with a layer of gold to increase

conductivity. The samples were observed at 5 Torr and 7 °C.

Mettler thermal analyzers having 851e TG and 821e DSC modules were utilized for

the thermal characterization of the polymers. DSC scans were recorded under a

nitrogen atmosphere (flow rate =80 mL/min) at a heating rate of 10°C ·min-1. The

sample amount was about 10mg and an aluminum crucible (40mL) was used. The

thermal stability was determined by recording thermogravimetric (TG) traces in a dry

air atmosphere (flow rate = 50 mL/ min) using powdered samples in open aluminum

oxide crucibles (70mL). A heating rate of 10 °C ·min-1 and a sample size of 10-12mg

were used in each experiment. The STAReSW 9.20 software (Mettler) was used for

data recording and interpretation.

X-ray diffraction pattern of the polymer films (100 mm thick) was recorded with a

Siemens goniometer D5000 using Cu Ka-radiation, of λ=1.54 Å.

A Zwick/Roell BT1-Fr0.5TN-D14 machine equipped with a 200 N KAF-TC load

sensor was used to determine the mechanical properties of the polymer films.

Bone-like specimens were punched out of the polymer with an average length of 1.4

cm and a width of 0.20 cm. The thickness of the films was measured by micrometer in

different position. A preload of 0.1 MPa and a subsequent traction speed of 50

mm/min were applied. 5 samples were measured for each composition. DMTA

measurements were performed on a “Dynamic Mechanical Thermal Analyzer” by PL

Thermal Sciences. The acquired data were evaluated with the associated software

Plus5 5.2 by PL Thermal Sciences; vibration frequencies of 10, 1 and 0.1 Hz were

applied for analysis.

For degradability studies, weight loss was calculated by the following equation,

Weight loss % = (W0 - Wt) / W0

Where W0 and Wt represent respectively the dry weights of the films before and after

degradation.

108  

2.4. Enzymatic degradation

Enzymatic degradation experiments were carried out at 37 °C in a 0.05 M, pH 7.0

phosphate buffer. Square samples with dimensions of 10×10×0.2mm were cut from

the films of polyester and placed in vials containing 5ml of buffer solution with 1 mg

of pseudomonas lipase and 1 mg of sodium azide. At predetermined time intervals,

the degradation medium was filtered by disposable filter (pore size 0.5 µm). The

filtered solid was washed with distilled water, and then vacuum-dried at room

temperature for 2 days before analysis (GPC and NMR). The filtrate (solution) was

evaporated by freeze-drying and the residue was analysed by GPC and 1H NMR.

3. Results and Discussion

An all-odd aliphatic polyester-5,7 (poly(pentylene heptanoate)) was made by

condensation polymerization of 1,5-Pentanediol and 1,7-heptandioicacid (pimelic acid)

in the presence of titanium tetrabutoxide (TBT) catalyst. The reaction was carried out

in two steps and polyphosphoric acid was used as heat stabiliser (Scheme 1). The

weight average molecular weight of the polyester-5,7 as determined by gel

permeation chromatography in THF was about 23,000 with polydisperisty of 1.8. The

structural characterization of the resulting polymer was done using NMR

spectroscopy. The representative 1H NMR spectrum of the resulting polymer with

peak assignments is shown in the Fig. 1. The peak appearing at δ=2.24 ppm (proton 4)

was assigned to –C(O)CH2- protons of pimelic acid part and the peak at δ=4.0 ppm

(proton 1) originated from -OCH2- protons of di alcohol part. Other peaks from

methylene protons of diacid and diols parts were observed as overlapping peaks

centered at δ=1.59 ppm and δ=1.34 ppm in 1H NMR but were splitted into well

resolved peaks in 13C NMR spectrum. To further confirm the structure of the

polyester and for the correct peak assignments in 1H and 13C NMR spectra, the 2D

NMR techniques like HMQC (Heteronuclear Multiple Quantum Correlation) and

HMBC (Heteronuclear Multiple Bond Correlation) were used. HMQC (Not shown

here) helped in unambiguous assignment of carbons 1 and 4 and in correlating other

109  

methylene peaks in 1H NMR to 13C NMR peaks. The correct and unambiguous peak

assignments of other peaks were done by seeing corresponding correlations in HMBC

NMR spectrum (Fig. 2). Proton 1 as expected showed 3 cross-peaks (2 and 3 bond

correlations with carbons 2,3 and the carbonyl carbon 7 respectively). Further,

methylene protons in the lower ppm region were also distinguished by seeing

expected correlations in HMBC NMR spectrum as shown in the Fig. 2.

Scheme 1. Synthetic scheme for the formation of polyester-5,7 by polycondensation

of 1,7-heptanedioicacid and 1,5-pentanediol.

OH

OHO

OO

n

11

22

3 2' 2'

3' 44

Figure 1. 1H NMR spectrum of polyester-5,7 in CDCl3.

The thermal stability of the polyester as checked using thermogravimetric analyzer

and was found to be well above 300 oC. One of the measures of thermal stability, T5%

(temperature at which 5% weight loss takes place) was very high, of about 360 oC.

HO

O

OH

O

HO OH

HO

O

O

O

On

H

TBT, PPA,190-230 °C

1 bar...0,2 mbar

ppm (t1)

1.02.03.04.05.06.07.0

1

2 + 2'

3 + 3'

4

solvent

110  

Differential scanning calorimetry was used for determining the phase transitions like

glass transition and melting. A single melting peak was seen at about 43 oC. No clear

glass transition could be seen in DSC. In an attempt to observe glass transition

temperature, different DSC scans were run by changing the amount of the sample and

heating rates but still no clear DSC was seen. Further dynamic mechanical thermal

analysis (DMTA) on polyester films were carried out for seeing the glass transition at

different frequencies. The DMTA curves are shown in the Fig. 3. DMTA showed very

broad transitions and therefore could not give accurate glass transition temperature of

the polyester. Hence, a new tool for determining the glass transition temperature

was employed: non-contact, non-destructive THz TDS [23].

Figure 2. 2D 1H-13C HMBC (Heteronuclear Multiple Bond Correlation) of

polyester-5,7.

Temperature-dependent THz TDS measurements reveal the glass transition by a

change in the thermal gradient of the THz refractive index. This step marks the

beginning of the translational motion of backbone chain segments in the amorphous

domains. Due to the model of the free volume, there is a decrease in density with

ppm 1.001.502.002.503.003.504.00

50

100

150

ppm

1 4 2+5 3+6

1

7

426

35 (1,3)

(1,2)

(1,7)

(4,5)(4,6)

(4,7)

(2,1)

(5,4)(5,6)

(2,3) (6,5)(6,4)

(3,2)

(3,1)

(5,7)

111  

increasing temperature that is higher at temperatures above the glass transition than

below. This two regime behavior is reflected by the refractive index according to the

Lorentz-Lorenz law [24]. Figure 4 depicts the temperature-dependent refractive index

of the biodegradable polyester-5,7 at 1.0 THz (33 cm-1). Both temperature regimes

can be fitted by a linear regression. The intersection yields the glass transition

temperature Tg in the vicinity of -53 °C. This novel method still succeeds when

conventional techniques, such as differential scanning calorimetry or

dynamic-mechanical analysis, fail, e. g. in case of highly-crystalline samples [25].

Figure 3. Dynamic mechanical thermal analysis of polyester-5,7 at different

frequencies showing broad transitions.

112  

Figure 4. THz refractometry reveals the glass transition temperature at the

intersection of two linear fits representing the two temperature regimes of the

refractive index at 1.0 THz (33 cm-1).

 

Mechanical testing showed very low elongation at break of around 1.5%, moderate

tensile strength of about 4 MPa and E modulus of 0.36 GPa for polyester-5,7. The

known degradable polyester PCL shows tensile strength in the range 4-28 MPa

depending upon molecular weight. PCL of Mn = about 42,500 (Aldrich) showed a

tensile strength of about 13 MPa as tested by us. Although, the XRD study (Fig. 5) of

the polyester-5,7 revealed no additional diffraction profile distinct from those coming

from the semi-crystalline PCL (the strong diffraction rings derived from the (110) and

(200) planes were observed at 2θ = 21.4° and 23.8° respectively), a significant

difference in elongation behavior between PCL and polyester-5,7 showed difficult

crystallizability tendency of polyester-5,7 on stretching because of odd-odd carbon

atoms.

113  

0 20 40 60 80 100

0

2000

4000

6000

8000

10000

Inte

nsity

2θ (°)

PCL JM

Figure 5. XRD reflexes of PCL (Aldrich) due to the planes (110) and (200) at 2θ =

21.4" and 23.8" in comparison to the polyester-5,7.

The degradability of the polyester-5,7 was evaluated using lipase from P. Cepacia at

37 oC. The weight loss profile of polyester-5,7 at different time intervals in presence

of lipase enzyme is shown in the Fig. 6. A very fast degradation of polyester can be

seen from this data in comparison to the biodegradable PCL; after 8h in 0.2 mg/ml

pseudomonas lipase buffer solution, weight loss was already about 53%, after 22 h

more than 90 wt% of samples degraded to water soluble products which were further

analyzed by 1H NMR and GPC. The GPC of the degradation products in water after

different time intervals showed the presence of only low molecular weight fractions

(Fig.7a). Accurate molecular weights could not be determined as this range falls

below the calibration limit of the used GPC instrument. The 1H NMR spectrum of the

degradation products of polyester in water after 8 h of degradation is shown in the

Fig.8. The two new distinct peaks appeared at ppm 2.1 and 3.5 ppm which could be

attributed to the HOOC-CH2- and OH-CH2- end group protons and some new

overlapping peaks in the lower ppm region of the degradation products. The

degradation of the polyester is expected to give water soluble products like

114  

1,5-pentanedio, 1,7-heptanedioicacid and oligomeric polyesters with HO-CH2- and

HOOC-CH2- end groups. These end groups and 1,5-pentanedio, 1,7-heptandioicacid

were clearly seen in the NMR of the degradation product. The 1H NMR of

1,5-pentanedio, 1,7-heptanedioicacid is shown for comparison.

0 10 20 30 40 50 60 70 80 900

10

20

30

40

50

60

70

80

90

100

110

120

Wei

ght l

oss

(%)

Degradation time(h)

0 20 40 60 800

20

40

60

80

100

Wei

ght l

oss

(%)

Degradation time (h)

Figure 6. Weight loss profiles of A) polyester-5,7 and B) PCL; degradation in

phosphate buffer (pH 7.0) in presence of Pseudomonas lipase (0.2mg/ml).

The molecular weight of the solid product left after different time intervals of

enzymatic degradation is also followed by GPC and is shown in the figure 7b. There

was no significant change in the molecular weight and polydispersity index till about

22 h of enzymatic degradation. This gives a hint about the surface degradation

(A) 

(B) 

115  

mechanism which was further confirmed by monitoring the surface morphology as

described below. In comparison to the fast enzymatic degradation, the hydrolytic

degradation under physiological conditions did not show any weight loss within 3

weeks. This behavior is similar to polycaprolactone.

100 1000

0.0

0.2

0.4

0.6

0.8

1.0

RI s

igna

l (a.

u.)

Molar mass (g/ mol)

8 h 22 h 46 h

 

100 1000 10000 100000

0.0

0.2

0.4

0.6

0.8

1.0

rel.

inte

nsity

molar mass/ g × mol-1

0 h 8 h 22 h 46 h

 

Figure 7. (a) GPC curves for degradation products of polyester-5,7 at different time

intervals; degradation was done at 37 °C in pH 7.0 with Pseudomonas lipases (0.2

mg/ml).(b) Gel permeation chromatographic profiles of polyester-5,7 residues after

degradation at different time in pseudomonas lipase (0.2 mg/ ml).

(A) 

(B) 

 

Figu

37 °

Poly

calc

and

calc

PCL

for

and

the

degr

ure 8. 1H N

°C in pH 7.0

yester-5,7 b

culated base

the bigger

culated for P

L is taken f

about 8 h,

PCL to abo

degradation

raded samp

NMR of pol

0 with Pseu

before degr

ed on the h

r spherulite

PCL was ab

from the lit

the % crys

out 67-68%

n in the am

ple.

H

lyesert-5,7 a

udomonas lip

radation wa

heat of fusio

es as observ

bout 60% (M

terature) and

stallinity inc

% but with s

orphous reg

HO‐CH2‐ 

116 

after 8h of

pases (0.2 m

as semicrys

on observed

ved by opt

Melting ent

d smaller s

creased in t

smaller sphe

gion thereby

HOO

degradation

mg/ ml).

stalline wit

d in the firs

ical micros

thalpy of 13

pherulites (

the left ove

erulites for

y increasing

C‐CH2‐

n; degradati

h about 50

st heating c

scope. The

36 J/g for 1

(Fig. 9). Af

er samples o

PCL. This

g the % cry

ion was don

0% crystalli

ycle from D

% crystalli

00% crysta

fter degrada

of polyester

could be du

ystallinity of

ne at

inity

DSC

inity

alline

ation

r-5,7

ue to

f the

117  

Figure 9. Optical microscope pictures of polyester-5,7 (A) before degradation (B)

after degradation (8h) (C) polycaprolactone (PCL) before degradation and (D) PCL

after degradation (8h).

Figure 10. SEM pictures of polyester-5,7 and PCL films during degradation; (A)

Polyester-5,7 before degradation (B) after 8h (C) after 22h of degradation; (D) PCL

before degradation (E) after 8h (F) after 22 h of degradation.

(A)  (B)

(C)  (D)

(A)  (B) (C)

(D)  (E) (F)

118  

SEM was used to investigate the surface morphological changes during degradation.

The scanning electron micrographs of polyester before and after degradation are

shown in the Fig. 10. Degradation led to the surface erosion and fibrillar and sponge

like structures were observed. The SEM characterization of cross section of the

polyester film before and after 22 h degradation in lipase was also done. The

morphology after 22 h degradation remained same as before degradation, which

showed degradation was by surface erosion. The degradation behavior observed for

polyesters was almost same as observed for polycaprolactone except its fast rate of

enzymatic degradation.

4. Conclusions

Enzymatic polyester-5,7 based on pimelic acid and 1,5-pentanediol was successfully

synthesized by polycondensation in the melt. The chemical structure and composition

of the polyester was confirmed by 1D and 2D NMR techniques like 1H, 13C, HMQC

and HMBC. The polyester-5,7 was found to have similar crystal structure and

degradation mechanism as PCL but with fast degradation rate. The enzymatic

degradation products are mainly composed of low molecular weight oligomers and

diols and diacids. The degradation was started in the amorphous region and on the

surface with change in surface morphology. Initial studies regarding hydrolytic

degradation under physiological conditions showed similar trend like PCL and

detailed studies at different pH values are in progress. Such aliphatic polyesters are

highly promising for various applications where PCL is currently being used with the

advantage of having fast enzymatic degradation rate like degradable glue, packaging

etc. The degradation under conditions closer to animal organisms will come up later

on. Once the time of hydrolytic degradation under physiological conditions is also

established, it would be another addition to the class of degradable aliphatic

polyesters having its own profile for different biomedical applications.

119  

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8.4 Publication “Nanofibers by Green Electrospinning of Aqueous Suspensions

of Biodegradable BlockCopolyesters for Application in Medicine, Pharmacy and

Agriculture”

Jinyuan Sun, Kathrin Bubel, Fei Chen, Thomas Kissel, Seema Agarwal, Andreas Greiner*, Nanofibers by Green Electrospinning of Aqueous Suspensions of Biodegradable Block Copolyesters for Applications in Medicine, Pharmacy and Agriculture, Macromol. Rapid Commun. 2010, 31, 2077–2083

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8.5 Publication “Low dielectric constant polyimide nanomats by electrospinning”

Fei Chen, Debaditya Bera, Susanta Banerjee*, Seema Agarwal*, Low dielectric constant polyimide nanomats by electrospinning, Polymers for Advanced Technologies, 2011, early view online.

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